Russia said Monday it had ordered troops near the border with Ukraine to return to their bases, just days ahead of a presidential vote aimed at bringing the country out of deep crisis.

The move could ease tensions, but both Washington and NATO -- which noted it was the third time Moscow had made such a claim -- said they saw no sign of a withdrawal.

With a bloody insurgency raging in the east, the ominous presence of Russian troops across the border and an economy in deep recession, next Sunday's election will determine the very survival of Ukraine.

But it is unclear whether a large chunk of the population will want to -- or be able to -- turn out to choose a new president for a country many fear is on the brink of civil war.

Russian President Vladimir Putin heads to China on Tuesday to shore up eastern ties as relations with the West plunge to new lows over the Ukraine crisis.

During a two-day visit to Shanghai, Putin and Chinese host Xi Jinping will seek to clinch a raft of agreements including a landmark gas deal crucial for Moscow as Europe seeks to cut reliance on Russian oil and gas.

Leaders of Crimea's Tatar community on Saturday called off a ceremony to commemorate 70 years since their deportation by Stalin, after an official ban and fears of unrest.

Tensions have risen between local authorities and the Tatars -- Turkic-speaking Muslims who make up about 12 percent of Crimea's population -- since Russia's annexation of the Ukrainian peninsula in March.

Ukraine's election body issued a stark warning Saturday that it may be impossible to hold next week's presidential election in the east, where a pro-Moscow insurgency is threatening to plunge the country into all-out civil war.

Russia also questioned how an election taking place under the "thunder of guns" could possibly meet democratic norms and demanded that Kiev halt its offensive against the rebels.

Almost two million people in eastern Ukraine could be prevented from voting in the presidential election if the government fails to ensure security in rebel areas, the country's election body warned on Saturday.

The Central Election Commission said it could be impossible to organize the May 25 vote in the eastern regions of Donetsk and Lugansk, where pro-Russian insurgents fighting Kiev's rule have declared their own independent republics.

Hungarian Prime Minister Viktor Orban on Friday renewed a controversial call for autonomy for his ethnic kin in western Ukraine, less than a week after sparking a diplomatic row with both Kiev and Warsaw.

"The full weight of the Hungarian state is behind the autonomy demands of the Transcarpathian Hungarians," Orban said in an interview on the M1 public television channel late Friday.

Presidents Barack Obama and Francois Hollande warned Russia on Friday of significant new sanctions if Moscow keeps up its "provocative and destabilizing" behavior in Ukraine.

During their telephone call, the U.S. and French leaders also discussed the hunt for more than 200 kidnapped schoolgirls in Nigeria and a conference Hollande is hosting in Paris this weekend on combating the Islamic militant group Boko Haram that is holding them, the White House said in a statement.

U.S. Secretary of State John Kerry on Friday denounced fresh human rights abuses of Crimea's Tatars, 70 years after the minority group was expelled from the then-Soviet Union.

The Tatars, a Turkic-speaking Muslim group, will commemorate the anniversary of the mass deportation on Sunday amid tensions over Moscow's annexation of the peninsula, which has revived memories of the tragedy.

Armed rebels seized control of a Ukrainian military barracks in the insurgent-held eastern city of Donetsk on Friday, an Agence France Presse correspondent at the scene said.

Explosions were also heard in Donetsk, a rare occurrence in the region's main city in recent weeks despite the pro-Russian insurgency in the east.