GENEVA, April 17 - The United States, Russia, Ukraine and the European Union called after crisis talks on Thursday for an immediate halt to violence in Ukraine.

President Barack Obama said the meeting in Geneva between Russia and western powers was promising but that the United States and its allies were prepared to impose more sanctions on Russia if the situation fails to improve.

Russian President Vladimir Putin, speaking in Moscow, accused Ukraine's leaders of committing a "grave crime" by using the army to try to quell unrest in the east of the country, and did not rule out sending in Russian troops, Reuters reported.

Putin said he hoped he would not need to take such a step, and that diplomacy could succeed in resolving the standoff.

The comments came hours after separatists attacked a Ukrainian national guard base and Kiev said three of them were killed in the worst bloodshed yet in a 10-day pro-Russian uprising.

Ukrainian, Russian and Western diplomats were seeking to resolve a confrontation that has seen pro-Russian fighters seize official buildings across eastern Ukraine while Moscow masses tens of thousands of troops on the frontier.

"All sides must refrain from any violence, intimidation or provocative actions," a joint statement said.

"All illegal armed groups must be disarmed; all illegally seized buildings must be returned to legitimate owners; all illegally occupied streets, squares and other public places in Ukrainian cities and towns must be vacated," it added.

"There is the possibility, the prospect, that diplomacy may de-escalate the situation," Obama said at the White House, according to Reuters.

"The question now becomes, will in fact they use the influence they've exerted in a disruptive way to restore some order so that Ukrainians can carry out an election and move forward with the decentralization reforms that they've proposed," Obama said.

It was unclear if Russia would meet Western demands for it to stop stirring unrest in the east and withdraw its troops from the Ukrainian border. Moscow denies it is active in Ukraine.

"It will be a test for Russia, if Russia wants really to show willing to have stability in these regions," said Ukrainian Foreign Minister Andriy Deshchytsia.

The U.S. State Department said the talks had achieved more than some people had expected.

But a spokeswoman added: "But again, it's not a breakthrough until this is implemented on the ground, and we need to see the Russians follow up these words with actions."

There was skepticism over whether the agreement could work.

"Diplomacy cannot succeed if there is no room for compromise," said Ulrich Speck, a visiting scholar at Carnegie Europe. "The Kremlin is dedicated to get Ukraine under its control, one way or another. It feels that it has well advanced on that goal, and is not ready to back down. The West simply cannot agree to those conditions."

U.S. Secretary of State John Kerry said there would be additional sanctions on Russia if it did not act to calm tensions in Ukraine.

"If ... we don't see a movement in the right direction, then there will be additional sanctions, additional costs as a consequence," Kerry told reporters.

The United States and European Union have so far imposed visa bans and asset freezes on a small number of Russians, a response that Moscow has openly mocked. However, the Western states say they are now contemplating measures that could hurt Russia's economy more broadly.

But some EU nations at least are reluctant to press ahead with more sanctions, fearing that could provoke Russia further or end up hurting their own economies.

Kerry also took the opportunity to condemn as "intolerable" suggestions in the eastern city of Donetsk that Jews had been ordered to register with authorities.

Kerry and European Union foreign policy chief Catherine Ashton said the United States and the EU still had a significant difference with Russia over the status of Ukraine's Crimea region, annexed by Moscow last month. (rt/ez)