Secretary of State John Kerry threatened on Wednesday to end all cooperation between the United States and Russia to stop Syria’s civil war, unless Russian and Syrian government attacks on Aleppo end. More than 250 people are believed to have been killed in the besieged city in the last week.

Kerry’s warning came in a telephone call Wednesday to Russian Foreign Minister Sergey Lavrov, the State Department said, describing the latest U.S. ultimatum in Syria’s 5½-year conflict. Many have gone unfulfilled, including President Barack Obama’s declaration that the U.S. would take military action if Syrian President Bashar Assad crossed the “red line” of using chemical weapons. READ MORE