U.N. Syria envoy Staffan de Mistura vowed on Thursday to take the “last resort” option of airdrops and air lifts if there is no improvement in access to besieged areas in Syria by June 1, and did not rule out overriding the government’s objections.

He said without improved aid access and some restoration of Syria’s cessation of hostilities, the credibility of the next round of peace talks would be in question, but he would not abandon the talks, and was waiting for the right date.

De Mistura’s comments in Geneva came as Syrian regimes forces and allied militias penetrated an opposition bastion east of Damascus Thursday, in a swift advance that threatened to disrupt a years-long stalemate around the Syrian capital.

Air and missile-strikes pounded rebel positions in the eastern Ghouta suburbs, clearing the way for soldiers and Lebanon’s so-called Hezbollah militants to take the iconic opposition stronghold of Deir al-Asafir and other villages, according to the Syrian Observatory for Human Rights.

One of the most significant regime advances this year displaced hundreds of families, the UK-based Observatory said.

Syrian state media, which also reported many of the advances, said the army took advantage of rebel infighting in the eastern Ghouta pocket to make its strike. Around 500 militants have been killed in three weeks of infighting, according to the Observatory.

Opposition groups expelled government forces from Deir al-Asafir in 2012.

The Iran-backed Hezbollah has lost over 1,000 fighters defending President Bashar Assad’s administration, including many of its top leaders.

The militia blamed rebels for artillery shelling that killed its top military commander Mustafa Badreddine last week. He was killed near Damascus airport, close to eastern Ghouta.

Rebel groups have not claimed responsibility. The Observatory denied the reports of shelling.

Separately, southwest of Damascus, the Syrian army and its allies pressed attacks to try to shore up control of a main highway running from the capital to southwestern Syria, including Quneitra province and the Golan Heights, the Observatory said.

Government forces have carried out dozens of air raids around the town of Khan al-Shih, where rebel groups control areas straddling the highway, it said.

Syria’s war has killed 250,000 people, created the worst refugee crisis since World War II and allowed for the rise of ISIS.