The Syrian Observatory for Human Rights said barrel bombs -- crude, improvised munitions that cause indiscriminate damage -- were used, including on the town of Hammuriyeh, where 10 people were killed.
Earlier Syrian President Bashar al-Assad said his forces must push on with their campaign to retake the besieged enclave of Eastern Ghouta from rebels, despite mounting international calls to end the bloodshed.
He vowed to continue an offensive in eastern Ghouta near Damascus as his forces advanced into the last major rebel enclave near the capital.
The offensive is one of the deadliest in the war and one local insurgent group called it a “scorched earth” campaign.
The government is pressing on despite Western calls for it to abide by a 30-day, countrywide ceasefire demanded by the UN Security Council.
“We will continue fighting terrorism ... and the Ghouta operation is a continuation of fighting terrorism,” Mr Assad said in comments to journalists broadcast on state TV.
The advances have forced thousands of civilians to flee deeper into the rebel-held territory, where some 400,000 people live, the Syrian Observatory for Human Rights and a resident said on Sunday.
Government forces need to advance just a few more kilometres further to split the enclave in two, said a commander in the military alliance that backs Assad. The Observatory said government forces had seized a quarter of the territory.
Mr Assad said there was no contradiction between daily, five-hour humanitarian ceasefires called by his ally Russia, and ongoing combat operations, noting that advances by government forces in the last few days had occurred during the truce.
The Russian ceasefire plan calls for five-hour pauses to allow for aid deliveries and evacuations of civilians and wounded. The US State Department has called the Russian plan a“joke” and the White House on Sunday accused Russia of killing Syrian civilians.
Mr Assad, in his first comments on the offensive, said most people in Ghouta wanted to return to state rule.
“Therefore we must continue with the operation and in parallel open the way for civilians to leave,” he said.
Russia and Damascus have accused rebels of preventing civilians from leaving eastern Ghouta during the daily ceasefires. Rebels have consistently denied this accusation and say people will not leave because they fear the government.
A UN humanitarian official said people in eastern Ghouta were being subjected to unacceptable “collective punishment”, which is illegal under the Geneva Conventions.
Mr Assad dismissed Western statements about the humanitarian situation in eastern Ghouta as “a ridiculous lie”.
With the war entering its eighth year, capturing the eastern Ghouta area would be a major victory for Mr Assad, who has steadily recovered control of rebellious areas with Russian and Iranian support.
The White House, in its strongest accusations of Moscow’s complicity in the offensive to date, said on Sunday that Russian military aircraft carried out at least 20 daily bombing missions in Damascus and eastern Ghouta between February 24 and February 28.
“Russia has gone on to ignore (a UN ceasefire’s) terms and to kill innocent civilians under the false auspices of counter-terrorism operations,” the White House said in a statement, saying the Russian aircraft had taken off from Syria’s Humaymim Airfield.
French President Emmanuel Macron asked his Iranian counterpart Hassan Rouhani to put pressure on the Syrian government to end attacks against the Ghouta region and to allow humanitarian aid to flow.
British Prime Minister Theresa May agreed in a phone call with US President Donald Trump that Russia must use its influence to make Damascus cease the eastern Ghouta campaign, Mrs May’s office said.
Without decisive Western pressure to halt the offensive, eastern Ghouta appears on course to meet the same fate as other rebel areas retaken by Mr Assad, such as eastern Aleppo, recovered using similar tactics of siege, bombardment and ground assaults.
Rebels eventually withdrew from eastern Aleppo in late 2016 in a mediated deal, leaving to opposition-held territory near the Turkish border.
The multi-sided war, which has killed hundreds of thousands of people since 2011, has escalated on several fronts this year, as the collapse of Islamic State has given way to other conflicts between Syrian and international parties.
Hundreds of civilians killed
The Syrian Observatory for Human Rights said regime forces had advanced to within three kilometres of Ghouta's main town, Douma, after retaking "more than 25 per cent" of the enclave.
The advance on the back of 15 days of air strikes, artillery fire and rocket attacks that are reported to have killed more than 650 civilians, sent hundreds into flight to western parts of the enclave.
Everyone is on the road. There's destruction everywhere.
Syrian regime air strikes on the besieged rebel stronghold of Eastern Ghouta near Damascus killed more than 30 civilians including children on Sunday, the monitoring group said.
"Thirty-four civilians were killed in regime strikes and rockets on Eastern Ghouta," Rami Abdel Rahman, head of the SOHR, said - adding 11 of the victims were children.
Mr Rahman also said 26 of the people killed were in Ghouta's main town of Douma and its eastern suburb.
With the support of Russian warplanes, the Syrian military has advanced on several fronts, retaking control of farms and villages, a military source told state media.
The source said government forces seized a number of districts including Al-Nashabiyeh and Otaya, and had "eradicated terrorist groups" on the eastern outskirts of Damascus.
They have reached the centre of the enclave, to the edge of Beit Sawa, according to the Observatory.
Rebel group Jaish al-Islam shares control of rebel-held parts of Eastern Ghouta with Faylaq al-Rahman and Ahrar al-Sham.
Hamza Bayraqdar, a spokesman for Jaish al-Islam, tweeted the group's forces had launched "surprise attacks" against regime positions.
The Observatory, which relies on a network of sources on the ground, said rebels had retaken some parts of Shifoniya.
Hundreds flee
An AFP correspondent inside Eastern Ghouta saw hundreds of civilians on Sunday fleeing from the town of Beit Sawa in the southeast of the enclave.
The Observatory said some 2,000 civilians had fled regime shelling and clashes in eastern areas to western parts of the enclave.
"Everyone is on the road. There's destruction everywhere," said 35-year-old Abu Khalil, carrying a little girl in his arms wounded on the cheek.
Apart from the civilian losses, at least 76 pro-regime fighters and 43 rebels from Jaish al-Islam have also been killed in clashes since February 25, it says.
Encircled by regime-controlled territory and unable or unwilling to flee, Eastern Ghouta's 400,000 residents have in recent weeks suffered one of the most ferocious assaults of Syria's civil war.
Under siege since 2013, they had already been facing severe shortages of food and medicine. The region's over-burdened medical workers have been struggling to cope with the rising number of wounded.
While falling short of a 30-day ceasefire demanded by the United Nations, Russia's announcement of daily humanitarian pauses in fighting had raised hopes of some aid deliveries and evacuations.
A convoy of "46 truckloads of health and nutrition supplies, along with food for 27,500 people in need" would finally enter the battered enclave on Monday, the UN Office for the Coordination of Humanitarian Affairs said.
The OCHA said further deliveries would follow and that it had "approval" to help 70,000 needy residents.
Moscow has offered safe passage to non-combatants wishing to leave Eastern Ghouta during the pause, but no Syrian civilians have left the enclave since the first break in fighting took effect on Tuesday, the Observatory says.
Damascus and Moscow have accused rebels of preventing civilians from leaving.
'Simply unacceptable'
On the international front, US President Donald Trump and British Prime Minister Theresa May said Russia and Syria were responsible for the "heart-breaking human suffering" in Eastern Ghouta.
The two leaders, during a phone call, "agreed it was a humanitarian catastrophe, and that the overwhelming responsibility for the heart-breaking human suffering lay with the Syrian regime and Russia, as the regime's main backer", the premier's office said.
French President Emmanuel Macron called on his Iranian counterpart Hassan Rouhani to put the "necessary pressure" on Syria's regime to halt "indiscriminate" attacks on civilians.
Also in a phone call, Mr Macron underscored the "particular responsibility for Iran, because of its ties to the regime, regarding the implementation of the humanitarian truce" sought by the UN, his office said.
The UN's regional humanitarian coordinator for Syria, Panos Moumtzis, sounded the alarm over the increased violence.
"Instead of a much-needed reprieve, we continue to see more fighting, more death, and more disturbing reports of hunger and hospitals being bombed," he said.
"This collective punishment of civilians is simply unacceptable."
As Syria's conflict approaches its seventh anniversary, Mr Assad's forces, heavily backed by Russia, have retaken most of the territory once lost to rebels.
