"The secretary will continue to reaffirm the United States' unwavering support for Ukrainian sovereignty, unity, territorial integrity, and the right of the Ukrainian people to determine their own future, without outside interference or provocation by Russia," Psaki said.
The announcement came after Ukrainian First Deputy Prime Minister Vitaliy Yarema said Russia is massing more troops along the border.
"Today Russian troops are continuing to remain along the eastern border of Ukraine, constantly increasing their presence. The Armed Forces of Ukraine, in turn, have been put on full combat alert," Yarema told government officials in Kiev.
Thousands of troops wearing unmarked uniforms have been deployed to Ukraine's autonomous, pro-Russia Crimean peninsula in the weeks following the ouster of pro-Moscow Viktor Yanukovych from the Ukrainian presidency. Russian President Vladimir Putin has denied the troops are Russian, but they have been seen driving in military vehicles bearing Russian license plates and have been heard and recorded on video saying they're from Russia.
The Crimea Parliament scheduled a referendum Sunday on whether to secede from Ukraine and join Russia. The new government in Kiev, as well as international leaders, said the vote is illegal and would not be recognized.
Yarema said Ukrainian soldiers would not react to any provocation, Interfax-Ukraine reported.
"No provocation has been given a response," he said. "Not one conflict has continued for long. When our opponents saw that their plans didn't work, they started attracting mercenaries in order to inflame the situation from the inside."
Ukrainian Defense Minister Igor Tenyukh said 18,430 Russian troops were in Crimea as of Wednesday, with special forces being replaced by mechanized battalions, Lvivska Gazeta reported.
"We are aware of every step that's taken in Crimea -- where troops are being rotated, where military equipment is being moved to, what they are doing, and so on," Tenyukh said before a Cabinet meeting in Kiev. "For instance, Russia is implementing a troop rotation and bringing [special] forces from Chechnya into Crimea."
The Kiev Post reported Ukraine's large telecommunications services provider, Volia, ended its broadcast of Russian television channels -- ORT, RTR Planeta and NTV-Mir -- on its networks at the request of the National Council on Television and Radio Broadcasting.
The Post also reported Crimean authorities said they would nationalize property of Ukrainian state companies inside the autonomous region during the next few days. The officials said the nationalization effort would not affect private companies.