GENEVA (AP) — The United States and Russia promised to try to break the stalemate in Syria peace talks, a U.N. mediator said Thursday, as Syrian activists said government shelling and airstrikes with makeshift barrel bombs killed about 400 people in the country’s largest city so far this month.

A second round of peace talks in Geneva has offered a rare opportunity for conversation, but yielded little more than acrimony. The violence has escalated on the ground and delegates in Geneva have failed to even agree on an agenda for the talks.