Britain urged Russia on Saturday to calm the situation in Ukraine after Russian President Vladimir Putin secured his parliament's authority to invade his neighbor's territory.
"There can be no excuse for outside military intervention in Ukraine - a point I made to President Putin when we spoke yesterday," Prime Minister David Cameron said.
"Everyone must think carefully about their actions and work to lower, not escalate tension. The world is watching."
Britain called an emergency meeting of the United Nations Security Council for Saturday to discuss events in Ukraine.
Foreign Secretary William Hague described the Russian action as a "potentially grave threat" to Ukraine's sovereignty, independence and territorial integrity that breached a 1994 pact signed by Russia, the United States, Britain and Ukraine.
He 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.
"We condemn any act of aggression against Ukraine," he said.
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, the 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.
The agreement committed the signatory countries to refrain from using any force that threatened Ukraine's territorial integrity or political independence, never to use weapons against Ukraine except in self-defense, and to consult if any event arose to challenge these commitments.
Hague said international diplomatic action was needed to address the crisis and he had spoken to German Foreign Minister Frank-Walter Steinmeier on Saturday.
Hague also said he had spoken to Ukraine's acting president Oleksander Turchynov and made clear Britain's support for Ukraine's new government and its territorial integrity.
"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.
He said he would discuss with the Ukrainian government how Britain could support it in recovering improperly acquired assets.
Austria, Switzerland and Liechtenstein moved on Friday to freeze the assets and bank accounts of up to 20 Ukrainians including toppled president Viktor Yanukovich and his son after Ukraine's new rulers said billions of dollars had gone missing.
"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 Ukraine's southerly Crimean peninsula, where Russia bases its Black Sea fleet, immediately by any practical means.