Russia has vehemently denied allegations of helping the rebels in Ukraine, while acknowledging that Russian volunteers are fighting with the rebels. Western military experts say the sheer quantity of heavy weapons in rebel hands belies that claim.
Kerry came to Ukraine to show support for its embattled government as the Obama administration weighs sending arms to Kiev to help it fight Russian-backed separatists. The U.S. envoy brought $16.4 million in new U.S. humanitarian aid to Ukraine, but the government is anxious to reiterate its plea for lethal aid.
The Ukrainian military accused Russian special forces on Monday of taking part in attacks on the strategically important Donetsk airport in eastern Ukraine, where fighting has intensified in recent days despite a September ceasefire deal.
Kiev also accused Russia of smuggling ammunition for heavy artillery into the area to step up the assault.
Ukraine has repeatedly accused Russia of sending troops and military equipment to support rebels in the east, where pro-Russian separatism erupted in April. The Kremlin denies it is involved in the conflict, which has killed over 4,300 people.