The United States has urged the Syrian government to uphold its commitments to a nationwide ceasefire. But the White House also asserted that Germany's proposal to create safe zones on Syria's borders, to protect fleeing refugees, is not a viable alternative for now.

"The president is against safe zones, he's expressed that from the beginning because he doesn't want to put the United States in a situation of trying to enforce it. The focus of our military should be on degrading and destroying ISIL. We should, however, collectively as an international community, be conscientious about living up to the commitments that are made by all the parties in the context of a cessation of hostilities in those areas, where we've seen the cessation start to fray in recent weeks," said Josh Earnest, White House spokesman.

Washington and Moscow have agreed to station additional personnel in Geneva to help enforce a ceasefire. Both sides claim they are extending the truce to Aleppo, where government air strikes and rebel shelling has killed hundreds of civilians in the past week. Syria announced temporary local truces in two areas last week, but it did not include Aleppo.