The US president has said Russia has failed to abide by the spirit of a deal to ease tensions in Ukraine, and that new sanctions against Moscow were being “teed up”.
On Thursday, Barack Obama said that “malicious, armed men” continued to occupy buildings in eastern Ukraine - a reference to pro-Russian separatists who have taken over key state buildings in several towns near the border with Russia.
The continued occupation is against an deal agreed last week in Geneva by Ukraine, Russia, the EU and the US to defuse tensions in Ukraine.
"So far we have seen them not abide by the spirit or the letter of the agreement in Geneva," Obama said. If that continues, he said, "there will be further consequences and we will ramp up further sanctions."
However, Obama said he needed support from allies to make sanctions work against Russia and its president, Vladimir Putin.
"How well they change his calculus depends on the co-operation of other countries."
"It is important to emphasise that throughout this process our goal has been to change Mr Putin's calculus, that our preference is to resolve this diplomatically, that sanctions hurt Russia more than anybody else but they are disruptive to the global economy."
Ukraine said on Wednesday it was relaunching a military operation against the separatists, but so far no confrontations have been reported.
On the same day, Sergey Lavrov, the Russian foreign minister, gave warning that Russia would respond if its interests were threatened, and made reference to Russia’s invasion of Georgia in 2008 after Georgian forces attacked separatists in its pro-Moscow region of South Ossetia.
Lavrov's ministry later insisted that Ukraine withdraw its troops from the the southeast of Ukraine.
"The Russian side once again insists on the immediate de-escalation of the situation in Ukraine's southeast, the pullback of units of the Ukrainian army and the start of a genuine internal Ukrainian dialogue involving all of the country's regions and political formations," it said in a statement.
According to Western sources, Russia itself has 40,000 troops massed along the border with Ukraine, and 25,000 in the Crimea region, which Russia annexed last month.