U.S. Secretary of State Antony Blinken, in a phone call with Ukrainian Foreign Minister Dmytro Kuleba on Wednesday, affirmed Washington's support for Ukraine's territorial integrity "in the face of Russia's ongoing aggression," the State Department said in a statement.
Ukraine and Russia have been at loggerheads since Russia’s annexation of Crimea in 2014 and over its support for separatist rebels fighting in Ukraine’s eastern Donbas region, which Kyiv says has killed 14,000 people.
Blinken "expressed concern about the security situation in eastern Ukraine and offered condolences on the recent loss of four Ukrainian soldiers," the statement said.
Ukraine's armed forces said last week that four soldiers were killed in shelling by Russian forces in Donbas, the highest daily death total since a cease-fire agreement was reached last July.
Ukrainian commander-in-chief Ruslan Khomchak said on Tuesday Russia was building up armed forces near Ukraine's borders in a threat to the country's security.
The Kremlin said Wednesday it was concerned about mounting tensions in eastern Ukraine and that it feared Kyiv's government forces could do something to restart a conflict with pro-Russian separatists.