France and Britain called Thursday for the Syrian regime and its allies to end their “disastrous” siege on the city of Aleppo, saying Russia in particular has a unique ability to persuade the regime of Bashar Assad to end the war.

French Foreign Minister Jean-Marc Ayrault and his British counterpart Boris Johnson said in a joint statement after a meeting they held in Paris that the siege of the city where some 300,000 people are trapped “makes it impossible for peace negotiations to resume.”

The consequences of the siege, “including the bombardment of civilians and medical facilities, are already disastrous and could generate further refugees,” the statement said, according to Agence France Presse.

“The ministers solemnly called upon the Syrian regime’s allies to bring an immediate end to these operations which violate the truce agreed in Munich, and international law,” they said in the joint statement.

The ministers called for the cessation of hostilities agreement “to be fully and immediately restored, and for progress towards the establishment of a transitional authority with full executive powers.”

Syria’s U.N envoy Staffan de Mistura urged Russia and the United States to work together to end the war in Syria, saying the failure of their cooperation plan would have very bad repercussions on the peace talks that could resume soon.

De Mistura told reporters in Geneva that U.S. and Russian diplomats could visit the city soon to discuss the details of the plan.

De Mistura told reporters that he understood Russian military experts “and perhaps (some) from the U.S.” were headed to Geneva, “most likely in order to discuss the devils in the details” of the two powers’ efforts to end the fighting in Syria.

The diplomat said it was premature to comment on the announcement made by the Syrian regime and Russia on the plan to help citizens trapped in besieged areas east of Aleppo and give Amnesty to rebels who surrender.

The United Nations was not consulted on the issue, he said.

De Mistura added that the situation in Aleppo is extremely dangerous.