Russian Foreign Minister Sergei Lavrov has described Monday’s attack on a mobile hospital in Aleppo as a “planned action.” The shelling killed at least two Russian medical personnel.

“We are certain that this action on a mobile hospital, where Russian medics worked, was planned. And those who planned it are trying to keep their positions in Aleppo, while being supported by their backers,” Lavrov said.

Lavrov also accused the U.S. of backtracking on Tuesday’s planned talks in Geneva over a rebel pullout from Aleppo in order to “buy time” for those battling Syrian President Bashar al-Assad. He added Moscow had the impression that a serious discussion with the American partners was not working out.

Lavrov also said U.S. representatives changed the text of an agreement in the run-up to the talks. The meeting has been indefinitely delayed, as a result of the changes.