The leaders of six major Western states called on Wednesday for an immediate ceasefire in the Syrian city of Aleppo and condemned Russia and Iran for supporting the Syrian regime.

In a joint statement by President Barack Obama and leaders of Britain, Germany, Italy, France and Canada demanded that Syria’s regime address the humanitarian crisis by allowing U.N. aid in to eastern Aleppo.

“The most urgent goal remains an immediate ceasefire so that the United Nations can bring humanitarian aid to people in East Aleppo,” said the leaders in the statement released in Berlin.

They said the Syrian regime must agree to a four-point plan put forward by the U.N. “We urge the Syrian regime to do this immediately to end the terrible situation in Aleppo; we demand Russia and Iran to exercise their influence to make this happen.”

The statement said that those who commit war crimes should have no impunity, urging the U.N. to collect evidence to bring individuals to justice. “We are ready to consider additional restrictive measures against individuals or institutions that act for the Syrian regime or in its name.”