MOSCOW — President Vladimir V. Putin of Russia told Chancellor Angela Merkel of Germany on Thursday that Ukraine must remove its military from the southeastern region of the country to resolve the showdown there with pro-Russian militants who have seized several official buildings, the Kremlin said.

“Putin emphasized that it was imperative today to withdraw all military units from the southeastern regions, stop the violence and immediately launch a broad national dialogue as part of the constitutional reform process involving all regions and political forces,” the Russian government said in a statement.

Early Friday, separatist forces in Slovyansk reported that the city was attacked by Ukrainian government forces. A separatist commander reported that a Ukrainian military helicopter had been shot down on the southern side of the city and that a checkpoint had been attacked.