The East-West split re-opened by the crisis in Ukraine hardened Wednesday when President Barack Obama threw Washington's weight firmly behind Kiev in its stand-off with Moscow.
Obama welcomed Ukraine's interim premier Arseniy Yatsenyuk to the White House and appeared by his side as both leaders sternly warned Russia that Ukraine would not surrender its sovereignty.
He repeated that Moscow would face unspecified "costs" if Russian President Vladimir Putin does not back down, and rejected a bid to hold what he called a "slapdash" referendum in Crimea.
"There's another path available and we hope President Putin is willing to seize that path," Obama told White House reporters, sitting alongside Yatsenyuk after their talks at the Oval Office.
"But if he does not, I'm very confident that the international community will stand firmly behind the Ukrainian government."
Yatsenyuk thanked Washington for its support and declared: "We fight for our freedom. We fight for our independence. We fight for our sovereignty. And we will never surrender."
After a series of meetings in Washington, Yatsenyuk was to head to New York on Thursday for talks at the United Nations.
Breakaway leaders on Ukraine's Crimean peninsula, backed by Putin, plan to hold a referendum on Sunday to split from Kiev and come under Moscow's wing.
Russian troops backed by ad hoc local militias secured the territory in the chaotic days last month after Ukraine's former pro-Kremlin leader Viktor Yanukovych was overthrown by a street revolt.
- 'We will stand with Ukraine' -
Obama said he hopes the crisis can be resolved through diplomacy, but Ukraine and the West do not recognize the referendum and Moscow does not recognize the Kiev government.
The US leader appeared to suggest that Crimea's future is not set in stone, but that any change in status would be a matter for Ukrainian constitutional process after upcoming elections.
"There is a constitutional process in place and a set of elections that they can move forward on that, in fact, could lead to different arrangements over time with the Crimean region, but that is not something that can be done with the barrel of a gun pointed at you."
And he made plain where he stood on Russian troop movements.
"We have been very clear that we consider the Russian incursion into Crimea outside of its bases to be a violation of international law," Obama said, underlining the depth of the divide.
"And we have been very firm in saying that we will stand with Ukraine and the Ukrainian people in ensuring that territorial integrity and sovereignty is maintained."
Yatsenyuk said he was "ready and open" for talks with Russia, but warned: "We want to be very clear that Ukraine is and will be a part of the Western world."
During his trip to Washington, Yatsenyuk also plans to try to iron out details of a $35 billion (25-billion-euro) aid package he says his nation's teetering economy needs to stay afloat.
He met Christine Lagarde, managing director of the International Monetary Fund, who described their talks as "productive."
Yatsenyuk also visited the World Bank and briefed US lawmakers, trying to build a case that a defense of Ukraine is a defense of the world order.
"If Russia goes further, this will totally and entirely undermine the global security," he said.
- G7 warns Russia -
Ukraine has declared it will not recognize Crimea's referendum, and has heavyweight support from the United States and European Union, but admits it would be powerless to intervene militarily.
Putin's diplomatic isolation intensified when the G7 industrialized nations urged Russia "to cease all efforts to change the status of Crimea contrary to Ukrainian law and in violation of international law."
"The annexation of Crimea could have grave implications for the legal order that protects the unity and sovereignty of all states," said a joint statement from the G7 powers.
EU foreign ministers are to discuss punitive measures against senior Russian officials at a meeting on Monday.
European leaders will then meet at a March 20 to 21 summit to witness the signing of what German Chancellor Angela Merkel said would be an historic EU-Ukraine association agreement.
Yatsenyuk confirmed in Washington that Ukraine hopes to sign the deal soon, perhaps as early as next week.
It was Yanukovych's refusal to sign this deal in November -- he chose instead to agree closer economic and political ties with Moscow -- that sparked the deadly protests that eventually ended his rule.