Robert Menendez (Democrat-New Jersey), chairman of the Senate Foreign Relations Committee, encouraged the administration to exercise existing authority to "more aggressively implement" an executive order imposing sanctions.
The Democratic senator said that, while he recognized the importance of acting with Europe, the administration "should not hesitate to act unilaterally to support an independent Ukraine."
Republicans in the U.S. Senate have introduced their own bill introducing harsher sanctions against Russia, but it has failed to gain any Democratic co-sponsors.
Ukraine and Russia have accused each other of sending in armored vehicles across the border in recent days.
StateDep on reports of #Ukraine forces use of white phosphorus: "we don't have evidence it’s credible 2 back it up"
— Natasha Mozgovaya (@mozgovaya) June 13, 2014
I spoke on the phone with President #Putin about the situation in #Ukraine, details of the call are available here http://t.co/bwsWEs2R54
— José Manuel Barroso (@BarrosoEU) June 13, 2014
Suspect tanks (and rocket launchers) in east #Ukraine came from #Russia, says @marieharf. says move "unacceptable" & warns of new sanctions.
— Matt Lee (@APDiploWriter) June 13, 2014
Ukraine's Energy Minister Yuriy Prodan has proposed holding new talks on a gas-pricing dispute with Russia, starting tomorrow.
His ministry said Prodan made the proposal in a telephone conversation today with EU Energy Commissioner Guenther Oettinger.
Ukrainian Prime Minister Arseniy Yatsenyuk, meanwhile, told his government to start preparing for the possibility of Russia cutting off its gas supplies within the next three days.
Ukraine has said it is ready to start settling its gas bills if Russia agrees to charge $326 per 1,000 cubic meters of gas for 18 months.
Russia has said its "final price" is $385 per 1,000 cubic meters and threatened to cut off gas supplies to Ukraine if Kyiv does not pay state-controlled Gazprom $1.95 billion by June 16.
Sunset in Donetsk pic.twitter.com/B36dw6Wf4S
— Paul Gypteau (@paulgypteau) June 13, 2014
Just now on Russian state TV: With this tank, #Kremlin claims, #Ukraine invaded Russian territory pic.twitter.com/qkk083dZm2
— MareikeAden (@MareikeAden) June 13, 2014
Rasmussen's comments, in a statement today, came after Ukraine accused Russia of allowing pro-Russian separatists fighting government forces to bring in three tanks and other military vehicles across the border into eastern Ukraine.
President Petro Poroshenko told Russian President Vladimir Putin in their first official telephone conversation yesterday that the situation was "unacceptable."
Russia did not immediately respond to Kyiv's accusations.
Nevertheless, Putin's spokesman said today that the president had ordered the Foreign Ministry to launch a formal complaint with Ukraine after a Ukrainian armored vehicle was found on Russian territory, close to the border with Ukraine's Luhansk region.
The European basketball governing body, FIBA Europe, announced its decision to relocate EuroBasket 2015 from Ukraine in a statement today.
It did not say which country or countries would now host the biennial tournament.
However, FIBA Europe said it will "start negotiations immediately" with Ukraine on the possibility of the country hosting the 2017 championship.
Following Russia's annexation of Crimea from Ukraine in March, tournament director Markiyan Lubkivsky said that hosting the event was no longer possible, but Ukrainian and European basketball authorities kept delaying a final decision, hoping the political situation would stabilize.
Reuters: #NATO says #Russian tanks in #Ukraine would be serious escalation http://t.co/2cMUCLpHw5
— Kyiv Post (@KyivPost) June 13, 2014