US Vice President Joe Biden warned Moscow on Tuesday that Western pressure will only harden if Russia' "aggression" against neighbouring war-scarred Ukraine persists.

"If Russian aggression persist, the cost imposed on Moscow will continue to rise," Biden said in a rare appearance by a top Western official before the Ukrainian parliament.

"The US will maintain pressure until Moscow fulfils its (peace deal) commitments," he said. "Despite some de-escalation in violence, there can be no sanctions relief unless and until Russia meets all of its commitments under the (February) Minsk agreements."