Syrian government forces have seized about one quarter of the rebel-held enclave of eastern Ghouta in recent days, activists say.
The Britain-based Syrian Observatory for Human Rights said on March 4 that most of the captured areas were farmlands.
A spokesman for Jaish al-Islam, one of the main insurgent groups in Ghouta, said that the government's "scorched earth policy" forced rebel fighters to retreat and regroup.
State media earlier said the military advanced on several fronts, taking control of villages and farms in the besieged area just to the east of the capital, Damascus.
Hundreds of people were reported to have fled the bombardment by government forces.
Violence in eastern Ghouta has left more than 600 people dead since government forces and their allies escalated their offensive on the Damascus suburb on February 18, according to activists.
Neither a daily five-hour cease-fire ordered by Moscow, Syria's main ally, nor the ordering of a nationwide, 30-day truce by the UN Security Council have led to any humanitarian relief for the embattled enclave.
The UN says some 393,000 people are trapped in Ghouta.
The army has been accused of targeting civilians, but it says it is trying to liberate the area from those Damascus describes as terrorists.
U.S. President Donald Trump and British Prime Minister Theresa May agreed in a phone call on March 4 that Syria and its Russian backers bore responsibility for the "heart-breaking human suffering" in Ghouta, Downing Street said.
May's office said the two agreed that "Russia and others with influence over the Syrian regime must act now to cease their campaign of violence and to protect civilians."
Meanwhile, French President Emmanuel Macron urged his Iranian counterpart, Hassan Rohani, to put the "necessary pressure" on the Syrian government to halt "indiscriminate" attacks on civilians in the enclave.
Macron made the call during a phone conversation ahead of Foreign Minister Jean-Yves Le Drian's trip to Tehran.
The French president underscored the "particular responsibility for Iran, because of its ties to the [Damascus] regime, regarding the implementation of the humanitarian truce" sought by the UN, his office said.
Russia and Iran have given Syrian President Bashar al-Assad's government crucial support throughout the seven-year civil war in Syria, which began with a government crackdown on peaceful protests.
Moscow helped turn the tide of the conflict in Assad's favor by launching a campaign of air strikes in 2015 and stepping up its military presence on the ground.
With reporting by Reuters, AFP, and the BBC