US Secretary of State John Kerry on Saturday told Moscow it was risking regional peace and security by deploying troops to Ukraine and warned of deep damage US-Russia ties.
"Unless immediate and concrete steps are taken by Russia to deescalate tensions, the effect on US-Russian relations and on Russia's international standing will be profound," Kerry said in a statement.
Moscow's actions not only violated international accords but were also "a threat to the peace and security of Ukraine, and the wider region," he added.
Kerry also revealed he had held a joint conference call earlier in the day with his global counterparts "to coordinate on next steps."
"We were unified in our assessment and will work closely together to support Ukraine and its people at this historic hour," Kerry said, repeating a call for Moscow to withdraw its troops.
It was not immediate clear which foreign ministers had joined the conference call.
Kerry also sought to tackle Russian concerns over the political upheavals in Ukraine, saying that "from day one, we've made clear that we recognize and respect Russia's ties to Ukraine and its concerns about treatment of ethnic Russians.
"But these concerns can and must be addressed in a way that does not violate Ukraine's sovereignty and territorial integrity, by directly engaging the Government of Ukraine," Kerry said.
The top US diplomat said he had spoken with Ukrainian interim president Oleksandr Turchynov earlier Saturday.
Kerry had assured the Ukrainian leader the new government had the "strong support of the United States" and commended it "for showing the utmost restraint in the face of the clear and present danger to the integrity of their state, and the assaults on their sovereignty."