The United States is "actively considering" adding names of Russians involved in the incursion in Ukraine to the "Magnitsky list" of human rights offenders subject to visa bans and asset freezes, a US State Department official said on Thursday.
"We are actively considering adding new names," Deputy Assistant Secretary of State Eric Rubin said at a House of Representatives committee hearing on the crisis in Ukraine.
However, Russian President Vladimir Putin is not among those targeted, a senior Obama administration official said.
"It is an unusual and extraordinary circumstance to sanction a head of state, and we would not begin our designations by doing so," the official said.
Under a 2012 US law named after Russian lawyer Sergei Magnitsky, who died in prison, the United States targeted Russian officials involved in human rights abuses with visa bans and asset freezes. The State Department placed 18 Russian individuals on a public list of those affected, and a handful of other senior officials are on a list that was not made public.
Earlier on Thursday Ukraine's Prime Minister Arseny Yatseniuk warned Russia that he would take military force if the Russian intervention escalates any further, urging Russia to withdraw its military from Crimea.
"In case of further escalation and military intervention into the Ukrainian territory by foreign forces, the Ukrainian government and Ukrainian military will act in accordance with the constitution and laws," Yatseniuk said in Brussels.
"We are ready to protect our country," he said.
Ukrainian forces have so far not responded to the Russian takeover of the Crimean peninsula. But this could change if the Russian intervention escalated, he said.
Yatseniuk, who came to Brussels to discuss the crisis with the leaders of the 28 countries of the European Union, said the talks with EU leaders were only about political and peaceful means of resolving the conflict.
He also said that a decree making Crimea part of Russia was an illegitimate move and Crimea was and will remain an integral part of the country.
"This is an illegitimate decision and this so-called referendum has no legal grounds at all. That's the reason why we urge the Russian government not to support those who support separatism in the Ukraine," Yatseniuk told a news conference in Brussels following talks with EU leaders.
"Crimea is was and will be an integral part of Ukraine."
The crisis in Ukraine escalated on Thursday after the parliament in Crimea, which has effectively been seized by Russian forces, voted to join Russia.