Former Ukrainian prime minister Yulia Tymoshenko claimed on Monday that by "occupying" Crimea, Russia had not only declared war on Ukraine but also on Britain and the United States.

That is because on December 5, 1994, Ukraine, Russia along with Britain and the United States signed an agreement in which the three powers guaranteed the territorial integrity of the former Soviet republic in exchange for Kiev giving up nuclear weapons.

The Black Sea peninsula is currently under de-facto occupation by pro-Kremlin troops, a situation which has been embraced by the local Russian speaking population fearing Kiev's new authorities.

However, under the terms of the 1994 so-called Budapest memorandum the three major powers affirmed their commitment to respect the independence, sovereignty and existing borders of Ukraine. It was signed three years after Ukraine became an independent state.

Russia, the U.S. and Britain also agreed to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that none of their weapons would ever be used against Ukraine except in self-defense or otherwise in accordance with the Charter of the United Nations.

In the memorandum, they also agreed to seek immediate United Nations Security Council action to provide assistance to Ukraine if Kiev should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used.

The agreement rules out economic coercion and calls for consultations in the event of a situation that raises a question concerning these commitments.

Ahead of a scheduled meeting with Russian Foreign Minister Sergei Lavrov on Monday, UN Secretary General Ban Ki-moon called on Moscow to avoid actions that might worsen the situation in Ukraine.