Secretary of State John Kerry will be heading to Kyiv Monday for discussions there Tuesday, the U.S. top diplomat tweeted.
"Secretary Kerry will meet with senior representatives of Ukraine's new government, leaders of the Rada [Ukraine's parliament], and members of civil society. The Secretary will reaffirm the United States' strong support for Ukrainian sovereignty, independence, territorial integrity, and the right of the Ukrainian people to determine their own future, without outside interference or provocation," says a statement by the U.S. State Department.
Quoting a senior U.S. government official, the New York Times reports that the trip will be a gesture of support for the new Ukrainian government.
Meanwhile, Ukraine has ordered a full military mobilization after Russian lawmakers authorized the deployment of troops on Ukraine territory.
Ukraine's new Prime Minister Arseniy Yatsenyuk says the Russian decision is a declaration of war. The pro-Western government in Kyiv, which ousted pro-Russian President Viktor Yanukovich last month, has appealed to the international community for help.
Prime Minister Yatsenyuk said in an address from parliament Sunday "we are on the brink of disaster" and called for Russia to pull back its military and abide by international obligations. Up to one million army reservists are to report to duty on Monday.
In Washington, Secretary of State John Kerry on Sunday condemned Russia's "incredible act of aggression" in Ukraine and threatened economic sanctions by the United States and allies to isolate Moscow, but called for a peaceful resolution to the crisis.
"You just don't in the 21st century behave in 19th century fashion by invading another country on completely trumped up pre-text," Kerry told the CBS program "Face the Nation."
News agencies in Moscow report Russian President Vladimir Putin told U.S. President Barack Obama, in a Saturday telephone call, Moscow reserves the right to protect ethnic Russians in Crimea and eastern Ukraine.
Russian and pro-Russian forces appear in control of much of Crimea. The autonomous Ukrainian republic, home to Russia's Black Sea Fleet, has an ethnic Russian majority and has historic ties to Moscow.
Unidentified soldiers, widely believed to be Russian troops, took up positions around Simferopol. Ukrainian soldiers and police seen on the streets appeared to be following an order from the region's pro-Russian prime minister to fall under his command or resign.
Reports that Russian troops had surrounded several Ukrainian military bases on the peninsula added to the tensions, though there appears to be no direct conflict.
In a news conference Sunday, local parliament speaker Vladimir Konstantinov claimed the situation had "stabilized." He dismissed the legitimacy of the new government in Kyiv, characterizing it as composed of extremists.
In Kyiv, meanwhile, prosecutors have opened a treason case against the newly-appointed head of Ukraine's Black Sea fleet, after he renounced his post and swore allegiance to pro-Russian leaders in Crimea. Authorities said Admiral Denys Berezovsky, appointed Saturday, offered no resistance later in the day when his headquarters in Sevastopol were surrounded by Russian troops.
While the events in Crimea have as yet proved bloodless, fears are rising that in eastern Ukraine, with sizable factions both pro- and anti-Russian, any movement of Russian troops would provoke a far more inflammatory reaction.
NATO Secretary General Anders Fogh Ramussen warned Moscow on Sunday it was threatening peace in Europe with its seizure of Crimea. He said NATO supports the right of the Ukrainian people to determine their future. He said Ukraine is a "valued" NATO partner and urged Russia to "de-escalate" the tension in the region.
Ukraine withdrew its coast guard vessels from two ports in Crimea and moved them to other Black Sea bases on Sunday.
Ukraine's border guard said in a statement that vessels from the Crimean ports of Kerch and Sevastopol had been moved to Odessa and Mariupol. The situation on Ukraine's frontiers was stable apart from in Crimea, the statement said.
Putin has declared he has the right to invade Ukraine to protect Russian citizens. Russian forces have seized Crimea, where they have a naval base at Sevastopol, but have not entered other parts of Ukraine.
On Saturday Obama said the United States is suspending participation in meetings to prepare for the G8 economic summit later this year in Sochi, Russia.
Obama said the appropriate way to address this matter is by direct engagement with the Ukrainian government and through international monitors.
The Pentagon says Defense Secretary Chuck Hagel spoke with Russian Defense Minister Sergei Shoygu Saturday. Officials say Hagel told Shoygu that without a change on the ground, Russia is risking more regional instability, global isolation, and an escalation that would threaten European and international security.
NATO ambassadors met Sunday in Brussels to discuss Ukraine. A meeting of the NATO-Ukraine Commission was also scheduled.
In New York, the United Nations said now is the time for "cool heads to prevail."
Ukrainian Ambassador Yuriy Sergeyev told the Security Council that 15,000 Russian troops are already in Crimea under the pretense of protecting Russian citizens.
Russian Ambassador Vitaly Churkin blamed the West for ratcheting up tensions in Ukraine and backing protests that ousted President Viktor Yanukovych last month.
Russia has said its troop movements in Crimea, where it leases a naval base in Sevastopol, conform to agreements with Ukraine.
Crimea is a Black Sea peninsula placed under Ukrainian control in 1954 by then-Soviet leader Nikita Khrushchev. It became part of Ukraine when the Soviet Union collapsed in 1991. Crimea has a tiny border with Russia on its far eastern point. Most of the people living in Crimea are ethnic Russians, but the region also is home to ethnic Muslim Tartars who generally show disdain toward Russia.
In Moscow and St. Petersburg, thousands of people turned out for officially sponsored demonstrations supporting Russia's military thrust into Crimea.
Smaller anti-war demonstrations took place in both cities, resulting in about 300 arrests.
Public opinion polls in Russia indicate that majorities of people here believe that the Ukraine's largely Russian speaking peninsula should belong to Russia. Hints emerged Sunday as to Russia's goal in Crimea.
Russian state news agency reported that a referendum scheduled for March 30 in Crimea will give voters the choice of: independence, continued autonomy within Ukraine, or annexation by Russia.
On a parallel track, Russia's Duma is to debate next week a new law that would make it easier for Russia to absorb new territories. Under this legislation, local referendums would trump international treaties.