President Barack Obama has warned Russia against further encroachment into Ukraine.
Mr Obama said further moves would be a bad choice for Russian president Vladimir Putin and insisted there is another path available to Moscow.
Mr Obama was speaking at a news conference in the Netherlands amid a flurry of diplomatic activity in the Hague. He said world leaders have created the framework for additional costs to be imposed on Russia if it moves further into Ukraine.
The president said if Ukrainians had a choice they would seek to have a relationship with both Europe and Russia, adding it is not a zero-sum game.
Mr Obama also said the US was not recognising Russia's annexation of Crimea.
His comments came after politicians in Ukraine accepted the resignation of the defence minister as thousands of troops started withdrawing from the Crimean peninsula, now controlled by Russia.
In an address to parliament, Igor Tenyukh rejected criticism that he had failed to issue clear instructions to troops, but that he reserved the right to step down. Deputies initially refused his resignation but then accepted it.
Authorities in Ukraine have come under criticism for their often-hesitant reaction to Russia's annexation of Crimea, which was formalised following a hastily organized referendum this month.
Ukrainian soldiers today started their westward journey from Crimea, piling on to buses and leaving behind former comrades who saluted them from outside a base overrun by Russian forces.
Mr Tenyukh earlier said he had received requests to leave Crimea from about 6,500 soldiers and family members- meaning about two-thirds of the 18,800 military personnel and relatives stationed there are taking their chances in the peninsula newly absorbed by Russia.
The acting president of the fledgling Ukrainian government yesterday ordered troops to withdraw from Crimea and servicemen escorted by Russian-commanded troops left a marine base on buses from the eastern Crimean port town of Feodosia.
As they left, Ukrainian servicemen standing outside the base saluted them and a small group of people standing outside looked on.
Mr Tenyukh said: "More than 6,500 servicemen and family - 4,300 servicemen and 2,200 family members - wish to continue serving in Ukraine's armed forces and will be evacuated from the autonomous republic of Crimea."
He added that 18,800 Ukrainian troops and family members were stationed in Crimea before Russian annexed the region.
A majority in parliament later voted to appoint Colonel General Mikhail Kovalyov as Mr Tenyukh's replacement.
The troop withdrawals came as Russia demanded more autonomy for Ukraine's regions, after Russian foreign minister Sergey Lavrov agreed to the highest level meeting yet between his government and a representative of the new Ukrainian government.
The meeting took place on the sidelines of a nuclear security summit in the Hague.
Mr Lavrov told Ukrainian counterpart Andriy Deshchytsia that Russia continued to want constitutional changes in Ukraine that would give more autonomy to all its regions.
Russia is eager to retain its influence in Ukraine's Russian-speaking eastern regions and prevent Ukraine from joining Nato. It has pushed for the new Ukraine to become a loose federation - demands the new Ukrainian government has rejected.
Before the meeting, Mr Deshchytsia said his government feared a Russian military build-up near Ukraine's border. "The possibility of a military invasion is very high. We are very much worried about this concentration of troops on our eastern border."
Earlier today, Mr Obama sought to turn up the heat on Moscow by meeting a close ally of Mr Putin.
In a last minute addition to his schedule, Mr Obama sat down with Kazakhstan's president Nursultan Nazarbayev in a long room at the US embassy in the Hague, with the two countries' flags set up behind them.
US officials offered no details about the meeting's agenda, but Mr Nazarbayev is part of a Russia-centred economic bloc focused on Eurasia.
As the leaders wrapped up their meeting, the White House released a joint statement from Mr Obama and Mr Nazarbayev that did not address the Ukraine situation, but focused instead on bilateral co-operation on nuclear security and non-proliferation.
Kazakhstan is the second largest country by territory and economy to emerge from the former Soviet Union. Mr Nazarbayev has manoeuvred between Russia and the West during more than two decades in power. Kazakhstan's energy resources and strong economy give it some independence from Moscow.
The meeting came after Mr Obama and Western allies moved yesterday to indefinitely cut Moscow out of a major international coalition, including cancelling an economic summit Mr Putin was to host this summer.
Mr Obama huddled with the leaders of Britain, France, Germany, Italy, Canada and Japan for an emergency meeting of the Group of Seven. Ahead of their private talks, British Prime Minister David Cameron and German Chancellor Angela Merkel declared that the G7 would not join Russia this year for the annual meeting of the G8.
Mr Obama said: "We're not recognising what is happening in Crimea," and rejected "the notion that a referendum sloppily organised over the course of two weeks" would "somehow be a valid process".
"We also are concerned about further encroachment by Russia into Ukraine," Mr Obama said, adding that Mr Putin "just has to understand there is a choice to be made here".
He said there is no expectation that Russian forces will be dislodged from Crimea by force and added it would be dishonest to say there is a simple resolution to the crisis.
The president said world leaders should use legal, diplomatic and political arguments to pressure Moscow.