If this violation of international law continues, the resolve of the United States and its international partners remains firm, President Obama said, before calling on Congress to support the IMF lending money to Ukraine.
President Obama will now speak on Ukraine at 1:05 pm ET, according to the White House
The State Department has called the Crimea referendum completely illegitimate, saying it violates the Ukrainian constitution, according to Reuters.
International police agency Interpol said on Thursday it was reviewing a request by Ukrainian authorities for it to issue a so-called "red notice" for the arrest of Viktor Yanukovich, who was ousted as its president.
"A request by Ukrainian authorities for an Interpol Red Notice, or international wanted persons alert, for the arrest of Viktor Yanukovich on charges including abuse of power and murder has been received," the France-based agency said in a statement.
The request was received on March 5, it said.
The Crimean assembly's vote to join Russia on Thursday was coordinated with the two houses of Russian parliament, a Ukrainian foreign ministry official said on Thursday.
"All that is happening now - the decision by the Crimean Supreme Council and statements by the Federation Council and State Duma - are evidence that this is coordinated action," Interfax news agency quoted Yevhen Perebiynis as saying.
"Now the masks are off and we can see for what purpose it all began."
Crimea's parliament voted to join Russia on Thursday and its Moscow-backed government set a referendum for March 16.
Russia on Thursday dismissed as "primitive distortion of reality", cynicism and double standards a U.S. Department of State fact sheet that dismissed President Vladimir Putin's comments on the crisis in Ukraine as "false claims".
"It's clear that in Washington, as before, they are unable to accept a situation developing not according to their templates," Alexander Lukashevich, the Russian Foreign Ministry spokesman, said in a statement.
The U.S. is questioning the legitimacy of a referendum in Crimea on whether to break away from Ukraine and join Russia instead.
A senior administration official says Ukraine's "legitimate government" must be involved in any decisions about Crimea's future. The official says actions that do not include the government in Kiev are violations of international law.
Crimea is a pro-Russian area of Ukraine. Lawmakers there voted unanimously Thursday to hold a March 16 referendum to decide their allegiance. Russian forces moved into the Crimea peninsula last week after Ukraine's Moscow-backed president fled Kiev.
The official was not authorized to discuss the developments in Crimea by name and spoke on condition of anonymity.
[The Associated Press]
We will be restricting visas as well as pulling visas for people who already have them, a senior administration official said on the call.
The United States will notify anyone who holds a visa if their visa has been revoked, the official added.
Senior administration officials are currently giving journalists a phone briefing on the visa bans and executive order announced Thursday morning. The call began a little after 8:30 a.m. ET.
The order gives the administration flexibility to respond based on Russia's actions in the coming days, a senior administration official said on the phone.
The Polish defense minister says the OSCE mission on its way to Ukraine's Crimea was detained by men in uniform, according to Reuters.
The decree making Crimea part of Russia is already in force and Ukrainian troops still on its territory will be treated as occupiers and forced to surrender or leave, the Russian-controlled region's deputy prime minister said.
"The only lawful armed force on the territory of the Crimea is the Russian armed forces," Rustam Temirgaliev said.
"Armed forces of any third country are occupiers. The Ukrainian armed forces have to choose: lay down their weapons, quit their posts, accept Russian citizenship and join the Russian military. If they do not agree, we are prepared to offer them safe passage from the territory of Crimea to their Ukrainian homeland."
U.S. Secretary of State John Kerry met his Russian counterpart Sergei Lavrov on the sidelines of a Rome conference on Libya to continue discussions on Ukraine, a senior U.S. official said on Thursday.
It is the second meeting in as many days between Kerry and Lavrov, who met in Paris on Wednesday to talks about the crisis over the crisis in Ukraine's Crimea Peninsula.
Earlier, Kerry also met his counterparts from Britain, Germany, Italy and France to discuss the situation in Ukraine and to inform them of U.S. plans to sanction individuals and officials over events in Crimea.
According to Crimea's deputy prime minister, Ukrainian armed forces in Crimea, as forces of a third country, will be considered 'occupiers,' Reuters reports. They must surrender or leave the territory, the deputy prime minister reportedly stated.
President Barack Obama on Thursday ordered the freezing of U.S. assets and a ban on travel into the United States of those involved in threatening the sovereignty and territorial integrity of Ukraine.
Obama signed an executive order aimed at punishing those Russians and Ukrainians responsible for a Russian incursion into the Crimea region of southern Ukraine, a crisis that has raised old-style Cold War tensions.
The order, the White House said in a statement, is "a flexible tool that will allow us to sanction those who are most directly involved in destabilizing Ukraine, including the military intervention in Crimea, and does not preclude further steps should the situation deteriorate."
In addition, the State Department is putting in place visa restrictions on a number of officials and individuals, reflecting a policy decision to deny visas to those responsible for or complicit in threatening the sovereignty and territorial integrity of Ukraine.
According to a U.S. official, Secretary of State Kerry met with Russia's Lavrov on the sidelines of the meeting in Rome, Reuters reports.
The USS Truxtun, a U.S. Navy guided-missile destroyer, is heading to the Black Sea where it will conduct training with Bulgarian and Romanian naval forces that was scheduled long before the crisis in Ukraine, the Navy said on Thursday.
"While in the Black Sea, the ship will conduct a port visit and routine, previously planned exercises with allies and partners in the region," the Navy said in a statement.
"Truxtun's operations in the Black Sea were scheduled well in advance of her departure from the United States."
The ship is part of an aircraft carrier strike group that deployed from the United States in mid-February.
According to RIA, which is quoting a spokesman, Russian President Putin has discussed the Crimean parliament's appeal to let Crimea join Russia at a meeting of the security council, Reuters reports.
Ukrainian minister says the planned referendum on whether Crimea should become part of Russia would be illegitimate, according to Reuters.
Kerry says there are 'a number of ideas on the table' to resolve tensions between Russia and Ukraine, will consult with President Obama.
In response to a question from a reporter, Kerry says he had 'zero expectation' that Ukraine and Russian foreign ministers would meet in Paris.
Kerry adds that he expects to meet with Russian foreign minister Lavrov in Rome on Thursday.