Belarusian President Alexander Lukashenko says that Russia has agreed to offer security assistance in the case of external military threats.
Mr Lukashenko also voiced concerns over Nato military exercises taking place in neighbouring Poland and Lithuania.
The news comes as the embattled president faces mass protests over the disputed 9 August election.
Thousands gathered outside state television on Saturday, demanding full coverage of the demonstrations.
The unrest erupted after President Alexander Lukashenko claimed a landslide victory in last week’s election, the result of which has been condemned amid widespread allegations of vote-rigging.
The Central Election Commission says Mr Lukashenko, who has been in power since 1994, won 80.1% of the vote and the main opposition candidate Svetlana Tikhanovskaya 10.12%.
But Ms Tikhanovskaya insists that where votes were properly counted, she won support ranging from 60% to 70%.
What’s happening politically?
As the unrest continued on Saturday, Mr Lukashenko sought help from Russian President Vladimir Putin.
Mr Lukashenko said President Putin had promised to provide what he called comprehensive assistance in the event of external military threats to Belarus.
The announcement came the day after EU foreign ministers agreed to prepare new sanctions against Belarusian officials responsible for “falsification”. The US has also condemned the election as “not free and fair”.
In a joint statement on Saturday, meanwhile, the prime ministers of three Baltic republics – Latvia, Lithuania and Estonia – “expressed deep concern at the violent crackdown… and the political repression of the opposition by the authorities”.
Lithuania and Latvia have previously said they are prepared to mediate in Belarus, provided the authorities stopped violence against protesters and formed a national council with members of civil society. They warned that the alternative was sanctions.
The leaders said the presidential election was “neither free nor fair” and called for a “transparent” vote “with the participation of international observers”.
“The prime ministers urge the Belarusian authorities to refrain from violence against peaceful demonstrators [and to] release all political prisoners and those that have been detained,” the statement added.
Ms Tikhanovskaya left for Lithuania following the election after she publicly denounced the results. She had sent her children to Lithuania for safety before the vote.
Some 6,700 people were arrested in the wake of the election, and many have spoken of torture at the hands of the security services.
Amnesty International said accounts from released detainees suggested “widespread torture”.
What’s the latest with the protests?
Demonstrations have continued following Ms Tikhanovskaya’s call for further peaceful rallies on Friday.
Some 100 staff came out of the state television building to join Saturday’s protests, saying they planned a strike on Monday, AFP news agency reports. Others have signed a letter in support of a strike.
“Like everyone we are demanding free elections and the release of those detained at mass protests,” one employee, Andrei Yaroshevich, told AFP.
On election day, Belarusian state channels aired the voices of Lukashenko supporters and did not cover the demonstrations. State TV later showed footage of violence to blame protesters and warn people not to participate.
Several journalists have resigned over the coverage.
Earlier on Saturday, thousands of people waved flags, lit candles and laid flowers at the scene close to the metro station where one of the protesters, Alexander Taraikovsky, died on Monday.
Others held up pictures of injured protesters, while drivers joined in by honking their horns.
Many opposition supporters chanted “Leave!” – a call for President Lukashenko to resign – and some carried signs with slogans against police violence.
The circumstances of Mr Taraikovsky’s death are unclear.
Officials say he died when an explosive device went off in his hand during a protest, but his partner, Elena German, told the Associated Press news agency that she believed the 34-year-old was shot by police.
A “March for Freedom” is also planned in the centre of the city on Sunday, a week after the contested election.
More about the protests in Belarus