Secretary of State John Kerry, on the ground in Kiev during a hastily arranged visit to Ukraine, urged Russia to "return its troops to the barracks" and vowed to "stand with the people of Ukraine" if Moscow will not back down.
After the Obama administration announced a $1 billion loan guarantee for Ukraine while suspending military and other ties with Russia, Kerry said Tuesday that the U.S. is preparing with its allies to take "further steps" to "isolate" Russia.
Like other U.S. officials, he did not threaten military action, but warned that Russia would be shunned "politically, diplomatically and economically" if it does not "de-escalate" in Ukraine.
"We are not seeking confrontation," Kerry said.
But he blasted what he called Russia's "act of aggression" in the Crimean Peninsula, accusing the government of "falsehoods, intimidation and provocation." Appealing to Russian President Vladimir Putin's ambitions of being a world leader, he said the invasion at the "barrel of a gun" is not "21st century, G-8, major nation behavior."
Speaking at a separate event in Washington, President Obama also responded to a press conference Putin delivered earlier in the day. Obama disputed the notion that Russia has somehow been strategically deft during its confrontation.
"I actually think that this is not been a sign of -- of strength, but rather, is a reflection that countries near Russia have deep concerns and suspicions about this kind of meddling, and if anything, it will push many countries further away from Russia," he said.
The European Union's foreign ministers, meantime, issued a Thursday deadline for Putin to pull back his troops or face a rejection of visa liberalization and economic cooperation negotiations long in the works.
Western leaders point to huge hits that Russia's natural gas, uranium and coal industries would take if sanctions cut off exports to the EU, its largest customer.
Putin on Tuesday rejected the threat of sanctions, saying they will backfire against the West.
The U.S. and its allies acknowledge that few options exist beyond economic and diplomatic penalties, and critics said Obama administration efforts are too little, and too late, after years of trying to foster friendlier relations with Russia.
President Obama on Monday described the Russian advance as a violation of international law. He called on Congress to approve an aid package for the new Ukrainian government and repeated earlier threats that the U.S. will take steps to hobble Russia's economy and isolate it diplomatically if Putin does not back down.
The White House said Obama met for more than two hours Monday night with the National Security Council, including Secretary of State John Kerry and Defense Secretary Chuck Hagel, to discuss what steps the United States can take with international partners to further isolate Russia and persuade it to de-escalate the situation.
The Pentagon announced late Monday it was suspending military-to-military engagements with Russia, including exercises, bilateral meetings, port visits and conferences.
Some Republicans in Congress were considering a possible package of "debilitating economic sanctions" to get Putin's attention. House Foreign Affairs Committee Chairman Ed Royce said the U.S. and Europe should act collectively to threaten the Russian stock market, economy and ruble if Russia doesn't withdraw from Crimea.
"We can't just keep talking," Royce said. "We need to do something."
Speaking at a U.N. session in Geneva, Russian Foreign Minister Sergey Lavrov attempted to deflect blame back on the West. He defended the deployment of Russian troops in Ukraine as a necessary protection for his country's citizens living there.
"Those who are trying to interpret the situation as a sort of aggression and threatening us with sanctions and boycotts, these are the same partners who have been consistently and vigorously encouraging the political powers close to them to declare ultimatums and renounce dialogue," Lavrov said.
The Associated Press contributed to this report.