The self-proclaimed "Luhansk People's Republic" in eastern Ukraine said on Sunday it could soon hold a referendum on joining Russia, a move Ukraine said would have no legal basis.

"I think that in the near future a referendum will be held on the territory of the republic," local leader Leonid Pasechnik said, according to the region's news outlet. "The people will exercise their ultimate constitutional right and express their opinion on joining the Russian Federation."

"For some reason, I am sure this will be the case," he said.

"Fake referendums in the occupied parts of Ukraine are null and void. No country in the world will ever recognize the forceful change of Ukraine's internationally recognized borders," Ukrainian Foreign Ministry spokesperson Oleg Nikolenko said in a tweet.

Russian President Vladimir Putin on February 21 signed two decrees recognizing "the Luhansk People's Republic" and "the Donetsk People's Republic" as independent and sovereign states.

On February 24, Russia authorized "a special military operation" in Ukraine in response to the appeal of the leaders of the "republics" in the Donbass region.

Russia is trying to split Ukraine in two to create a Moscow-controlled region after failing to take over the whole country, Kyrylo Budanov, head of Ukrainian military intelligence said on Sunday in a statement.

He added that Ukraine would soon launch guerrilla warfare in what he called Russian-occupied territory.

Here are the latest developments in the Russia-Ukraine conflict:

Missiles strikes on Lviv

Four missiles hit Lviv on Saturday just 60 km from the Polish border, local officials said, in the most significant attack on the city in the month-old conflict.

Russia struck military targets in Lviv with high-precision cruise missiles, the Russian defense ministry said.

Ukraine forces recapture town

Ukraine said on Saturday its forces had recaptured the northeastern town of Trostianets, near the Russian border, one of the first towns taken by the Russian army.

Its defense ministry published images showing Ukrainian soldiers and civilians among heavily damaged buildings, and what appeared to be abandoned Russian military equipment along with a signpost to the town.

Kyiv curfew cancelled

The mayor of Ukraine's capital Kyiv on Saturday cancelled a curfew announced just hours earlier for the next day.

"New information from the military command: the Kyiv curfew will not enter into force tomorrow," mayor Vitali Klitschko announced on Telegram.

Russia denies calling up reservists

Russia denied on Saturday that it was planning to call up reservists, denouncing what it said were "false" summons to Russian men by Kyiv's security services.

"The Russian defense ministry is not summoning and does not plan to summon any reservists to the military commissariats," spokesman Igor Konashenkov said in a statement.

Ukraine president calls for weapons

In his latest video address, Ukrainian President Volodymyr Zelenskyy Saturday reiterated a call for planes while urging allies to supply Ukraine with more weapons.

"We need more ammunition. We need it to protect not only Ukraine but other Eastern European countries that Russia threatened to invade," he said.

"What is NATO doing? Is it being run by Russia? What are they waiting for? It's been 31 days. We are only asking for one percent of what NATO has, nothing more."

Biden's tough words

U.S. President Joe Biden said on Saturday Putin "cannot remain in power," drawing criticism from Moscow. A White House official later explained that the remark was not meant to support regime change in Russia. 

The Kremlin said Biden as a state leader should "keep their temper."

Over 3.7 million refugees

More than 3.7 million people have fled Ukraine since Russia's military action a month ago, the UN said on Saturday.

The UN's refugee agency, UNHCR, said 3,772,599 Ukrainians had fled the country – an increase of 46,793 from the previous day's figure. Around 90 percent of them were women and children. The UN estimated that another 6.5 million people had been displaced in Ukraine.

(With input from agencies)