Speaking to reporters at the White House on Monday, U.S. President Obama said Russia has violated international law with its military intervention in Ukraine.
"I think the world is largely united in recognizing that the steps Russia has taken are a violation of Ukraine's sovereignty, Ukraine's territorial integrity."
While Obama stressed the importance of de-escalating the situation, and called for international efforts, such as the deployment of international monitors, he also warned the U.S. had been examining a series of economic and diplomatic sanctions that will isolate Russia.
Following the president's remarks, the U.S. Trade Representative announced late Monday that it has suspended upcoming bilateral trade and investment talks with Russia.
U.S. Vice President Joe Biden is also urging Russia to pull its forces out of Ukraine.
In a phone call with Russian Prime Minister Dmitry Medvedev Monday, Biden called on Russia to engage in dialogue with the Ukrainian government immediately.
Meanwhile, it turns out Ukraine's ousted president Viktor Yanukovich requested Russian President Vladimir Putin use Russia's armed forces in Ukraine to restore law and order.
This is according to Russia's UN ambassador who read out a letter Yanukovich had written to Putin at an emergency meeting of the UN Security Council.
"Ukraine is on the brink of a civil war. The life, the security and the rights of people, particularly in the southeast part, in Crimea, are being threatened. So, in this regard, I will call on the president of Russia, Mr. Putin, asking him to use the armed forces of the Russian Federation to establish the legitimacy, peace, law and order, stability and defending the people of Ukraine."
Ukraine claims some 16-thousand Russian troops have been deployed in Crimea since last week.
Russian state media outlet Interfax earlier reported Russia's Black Sea Fleet had given Ukrainian forces until early Tuesday morning, local time to surrender or face attack.
Park Ji-won, Arirang News.