By Steve Holland and Aleksandar Vasovic
THE HAGUE/FEODOSIA, Crimea (Reuters) - U.S. President Barack Obama sought support from European allies and China on Monday to isolate Russia over its seizure of Crimea, and Ukraine told its remaining troops to leave the region after Russian forces overran one of Kiev's last bases there.
Obama, who has imposed tougher sanctions on Moscow than European leaders over its takeover of the Black Sea peninsula, will seek backing for his firm line at a meeting with other leaders of the G7 - a group of industrialized nations that excludes Russia, which joined in 1998 to form the G8.
Since the emergency one-hour G7 meeting on the sidelines of a nuclear security summit in The Hague was announced last week, President Vladimir Putin has signed laws completing Russia's annexation of the region.
White House officials accompanying Obama expressed concern on Monday at what they said was a Russian troop buildup near Ukraine and warned that any further military intervention would trigger wider sanctions than the measures taken so far.
In what has become the biggest East-West confrontation since the Cold War, the United States and the European Union have imposed visa bans and asset freezes on some of Putin's closest political and business allies. But they have held back so far from measures designed to hit Russia's wider economy.
"Europe and America are united in our support of the Ukrainian government and the Ukrainian people," Obama said after a meeting with Dutch Prime Minister Mark Rutte. "We're united in imposing a cost on Russia for its actions so far. Prime Minister Rutte rightly pointed out yesterday the growing sanctions would bring significant consequences to the Russian economy."
He also discussed the crisis at a private meeting with Chinese President Xi Jinping, whose government has voiced support for Ukraine's territorial integrity but refrained from criticizing Russia. The West wants Beijing's diplomatic support in an effort to restrain Putin.
Moscow formally annexed Crimea on March 21, five days after newly-installed pro-Moscow regional leaders held a referendum that yielded an overwhelming vote to join Russia. Kiev and the West denounced the annexation as illegal.
Western officials are now focused less on persuading Putin to relinquish Crimea - a goal that seems beyond reach - than on deterring him from seizing other parts of Ukraine.
"Our interest is not in seeing the situation escalate and devolve into hot conflict," White House national security adviser Susan Rice told reporters. "Our interest is in a diplomatic resolution, de-escalation, and obviously economic support for Ukraine, and to the extent that it continues to be necessary, further costs imposed on Russia for its actions."
In The Hague, leaders of the G7 - the United States, Japan, Canada, Germany, France, Britain and Italy - will discuss how to exert further pressure, and at what potential cost.
"The main idea for the G7 meeting is to show the isolation of Putin. We won't be adopting any sanctions but there might be discussion on what could be the next step," a G7 official said.
He said they were also expected to cancel plans for a G8 meeting at the Russian Olympics site in Sochi, for which preparations were put on hold after Moscow seized Crimea.
Persuading Europeans to sign on to tougher sanctions could be difficult for Obama. The EU does 10 times as much trade with Russia as the United States, and is the biggest customer for Russian oil and gas. The EU's 28 members include countries with widely varying relationships to Moscow.
Central and east European states, which once lived under Soviet domination and joined the EU in the last decade, are mostly urging caution out of fear for their own economies.
But German Chancellor Angela Merkel, the EU's most powerful leader, has taken a tough line with Putin and supported EU moves to reduce the bloc's long-term dependence on Russian energy.
So far, the seizure of Crimea has been largely bloodless, apart from one Ukrainian soldier and one pro-Moscow militia member killed in a shootout on Tuesday last week. Ukraine's troops left behind in Crimea have been besieged inside bases while offering little resistance.
Russian troops forced their way into a Ukrainian marine base in the port of Feodosia early on Monday, overrunning one of the last remaining symbols of resistance.
"Yesterday we had an agreement: we would lower our flag and the Russians would raise theirs. And this morning the Russians attacked, firing live ammunition. We had no weapons. We did not fire a round," said one marine, Ruslan, who was with his wife Katya and 9-month-old son.
Troops hugged each other in farewell. Some chanted "Hurra! Hurra!" in defiance. One marine in full uniform who declined to identify himself wept and blamed the government in Kiev for the chaotic end to the standoff.
In Kiev, acting president Oleksander Turchinov told parliament the remaining Ukrainian troops and their families would be pulled out of the region in the face of "threats to the lives and health of our service personnel".
That effectively ends any Ukrainian resistance, less than a month since Putin claimed Russia's right to intervene militarily on its neighbor's territory.
Although Russian forces have not entered other parts of Ukraine, NATO says they have built up at the border. The Western military alliance also fears Putin may have designs on a part of another former Soviet republic, Moldova.
Despite the disruption to East-West relations, Washington wants other diplomatic business with Moscow to continue.
U.S. Secretary of State John Kerry was to hold talks later on Monday with Russian counterpart Sergei Lavrov, after meeting the head of the Organisation for the Prohibition of Chemical Weapons. The OPCW is overseeing the destruction of Syria's toxic stockpile in action sponsored jointly by Washington and Moscow.
Russia hit back symbolically at Canada, announcing personal sanctions against 13 Canadian officials in retaliation for Ottawa's role in Western sanctions so far. It has already taken similar measures against senior U.S. Congress members but not yet European officials.
Western governments are struggling to find a balance between putting pressure on Putin, protecting their own economies and avoiding triggering a vicious cycle of sanctions and reprisals.
Rutte, who is making his residence available to Obama and the other G7 leaders for the talks on the sidelines of a nuclear security summit, said the West might want to move slowly.
"Russia has an economy that is highly focused on oil and gas," Rutte told Reuters. "If it came to putting in place sanctions, that would hurt Russia considerably. So in my view we should do everything to prevent that."
U.S. officials say any further sanctions will need to be carefully calibrated to avoid bans on entire sectors, such as oil or metals, that could reverberate through the global economy. Europe gets around a third of its oil and gas from Russia.
(Additional reporting by Jeff Mason and Justyna Pawlak in The Hague, Gabriela Baczynska in Simferopol, Natalia Zinets in Kiev; Writing by Paul Taylor; editing by David Stamp)