LONDON (Reuters) - British Foreign Secretary William Hague said on Saturday that Russian action was a "potentially grave threat" to Ukraine's sovereignty, independence and territorial integrity.
Hague said he had spoken to Russian Foreign Minister Sergei Lavrov to urge steps to calm the situation, and had sent a summons to the Russian ambassador to register Britain's concern.
"I am deeply concerned at the escalation of tensions in Ukraine, and the decision of the Russian parliament to authorize military action on Ukrainian soil against the wishes of the Ukrainian Government," Hague said in a statement.
"We condemn any act of aggression against Ukraine."
Hague, who is due to go to Kiev on Sunday, said Britain supported the Ukrainian government's request for urgent consultations in accordance with the 1994 Budapest Memorandum, signed by Britain, United States, Russia and Ukraine.
The memorandum provided guarantees of Ukraine's sovereignty and integrity in exchange for a Ukrainian commitment, since fulfilled, to give up its Soviet-era nuclear weapons.
Hague said he had spoken to Ukraine's acting president Oleksander Turchynov and made clear Britain's support for Ukraine's new government.
"I assured him of the UK's commitment to working with other international partners and institutions to ensure that reforms by Ukraine are matched by international willingness to provide economic support," Hague said.
Hague said international diplomatic action was needed to address the crisis.
He welcomed the calling of a United Nations Security Council meeting on Saturday in New York and backed a proposed emergency meeting of EU foreign ministers in Brussels on Monday.
He said he would discuss with the Ukrainian government how Britain could support it in recovering improperly acquired assets.
"The EU must agree urgently an asset freezing regime to target those suspected of laundering the proceeds of corruption," Hague said.
He advised all British nationals to leave Crimea immediately by any practical means.
(Reporting by Belinda Goldsmith; Editing by Kevin Liffey)