A top US official hit back on Friday against accusations the West has failed to live up to promises made exactly 20 years ago to Ukraine in exchange for it giving up nuclear weapons.

The Budapest Memorandum that came into force on December 5, 1994 led to Ukraine voluntarily giving up the huge stockpile of nuclear warheads it inherited from the Soviet Union.

In exchange, Britain, the United States and Russia agreed to respect Ukraine's borders and ensure its security -- a promise which many Ukrainians see as unkept in the wake of Russia's annexation of Crimea and support for eastern separatists this year.

But on a visit to Kiev on Friday, Rose Gottemoeller, head of arms control and international security for the US government, said her country "has gone every step towards continuing to defend and develop a means of bolstering Ukraine".

Gottemoeller, who was a negotiator at the Budapest talks 20 years ago, sidestepped questions from reporters on whether Ukraine would have been spared a Russian invasion if it still had nuclear weapons.

"We regret of course that Russia has not lived up to its commitments," she said.

But she added that the failure to rid Ukraine of atomic bombs "would have contributed to a very unstable 20 years not only in Ukraine's history but also in the history of the entire world because it would have placed a barrier in the way of further nuclear disarmament elsewhere."

She also emphasised that steps to reduce nuclear stockpiles in Russia and the United States had not been affected by the crisis over Ukraine, which has plunged East-West relations to levels not seen since the Cold War.

The two countries have continued to implement a so-called New START agreement signed in 2011 under which they have to limit their number of deployed strategic nuclear warheads to 1,550.