WASHINGTON - The United States said on Monday it is 'gravely concerned' by fighting in and around Debaltseve in eastern Ukraine that flared up after a ceasefire agreement and said it is closely monitoring reports of a new column of Russian military equipment moving toward the region. 'These aggressive actions and statements by the Russia-backed separatists threaten the most recent ceasefire,' the US State Department said in a prepared statement. 'We call on Russia and the separatists it backs to halt all attacks immediately.'