WASHINGTON (AP) — Secretary of State John Kerry agreed Monday to meet with top diplomats from Russia, Ukraine and the European Union in a new push to calm tensions in eastern Ukraine, as the White House threatened further sanctions if Moscow intervenes.

Kerry’s meeting with his counterparts will take place in the next 10 days, the State Department said, and comes amid mounting U.S. concern that Russia could build on its recent annexation of Crimea by trying to split off other areas of Ukraine. To that end, the White House issued a warning to Russian President Vladimir Putin.