VILNIUS, Lithuania (AP) — Issuing an outright warning to Moscow, Vice President Joe Biden declared Wednesday the United States will respond to any aggression against its NATO allies, as Russia’s neighbors looked warily to the escalating crisis in nearby Ukraine.

Standing side by side with a pair of Baltic leaders in Vilnius, Lithuania, Biden said the U.S. was “absolutely committed” to defending its allies, adding that President Barack Obama plans to seek concrete commitments from NATO members to ensure the alliance can safeguard its collective security.