BEIRUT/GENEVA (REUTERS, AFP) - Syrian President Bashar al-Assad vowed to continue an offensive in eastern Ghouta near Damascus on Sunday (March 4) 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," 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 (SOHR) 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 SOHR said government forces had seized a quarter of the territory.

Syrian regime air strikes on Sunday killed more than 30 civilians including children.

"Thirty-four civilians were killed in regime strikes and rockets on Eastern Ghouta," said Rami Abdel Rahman, head of SOHR, adding that 11 of the victims were children.

Rahman also said 26 of the people killed were in Ghouta's main town of Douma and its eastern suburb.

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.

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.

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 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 Feb 24 and Feb 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, 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 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.

SCORCHED EARTH

One of the main insurgent groups in eastern Ghouta, Jaish al-Islam, said the government's "scorched earth policy" had forced rebels to retreat and regroup, but vowed to recover lost territory.

One resident estimated that thousands of people were on the move and seeking shelter in areas further from the frontlines.

The Observatory estimated that between 300 to 400 families, which is likely several thousand people, had fled areas seized by government forces since Saturday. The pro-Assad commander said civilians were fleeing to the town of Douma.

The Observatory says government shelling and air strikes have killed 659 people in eastern Ghouta since Feb 18, while rebel shelling of Damascus has killed 27.

UN regional humanitarian coordinator Panos Moumtzis said violence has escalated in eastern Ghouta and mortars fired into Damascus had killed and injured scores of civilians.

"Instead of a much-needed reprieve, we continue to see more fighting, more death, and more disturbing reports of hunger and hospitals being bombed," Moumtzis said in a statement.

The United Nations later issued a statement saying it had received approval for a 46-truck convoy to the Ghouta town of Douma on Monday, carrying health and nutrition supplies, and food for 27,500 people. A second trip was planned for Thursday.

Earlier, a UN official in Syria told Reuters a humanitarian convoy carrying life-saving supplies would not enter eastern Ghouta as had been planned on Sunday, citing a lack of permission.

DISTURBING REPORTS

Moumtzis also expressed concern about the situation in Afrin, a Kurdish region under Turkish assault since January, saying there were "disturbing reports" of civilian deaths and injuries and restrictions on civilian movement.

Turkey, backed by Syrian militias, has gained ground in recent days in Afrin. The Observatory said the advancing forces could soon besiege Afrin city, where 1 million people live.

The Observatory said Turkish forces had advanced to within 12km of Afrin.

Turkey's army said on Sunday it captured seven settlements including the town of Sheikh Hadid. Turkish forces have taken control of the roads from Rajo and Jandaris to Afrin, it also said in a statement.

Turkey has rejected Western calls for it to suspend the Afrin assault in line with the U.N. ceasefire, which does not apply to Islamic State, al Qaeda and groups associated with it, or other groups deemed terrorists by the Security Council.

Turkey views the YPG as an extension of the Kurdistan Workers' Party (PKK), which has fought a three-decade insurgency in Turkey and is deemed a terrorist group by the United States, the European Union and Turkey. The YPG has been an important ally for the United States in the fight against the Islamic State in Iraq and Syria.