Denis Pushilin and Leonid Pasechnik, the heads of the Donetsk and Luhansk people's republics, requested help "in repelling the aggression of the Ukrainian armed forces,"  the Interfax news agency quoted Kremlin spokesman Dmitry Peskov Peskov as saying.

The appeal raises the prospect of Russia's direct military involvement in eastern Ukraine.

For the latest news on the crisis in eastern Ukraine, click here for live updates.

Fears that Russia is planning an imminent offensive against its neighbor have heightened after President Vladimir Putin received authorization to use military force outside his country.

Meanwhile, Ukrainian lawmakers approved President Volodymyr Zelensky's decree imposing a state of emergency. The measure allows authorities to impose curfews and restrictions on movement, as well as ban political parties and organizations "in the interest of national security and public order."

US and Ukrainian reactions

Ukraine's Foreign Minister Dmytro Kuleba requested an urgent meeting of the UN Security Council regarding the appeal of the pro-Russian separatists.

"Ukraine has requested an urgent meeting of the UN Security Council due to the appeal by Russian occupation administrations in Donetsk and Luhansk to Russia with a request to provide them with military assistance, which is a further escalation of the security situation," Kuleba said on Twitter.

The White House said in response to the rebels' request that it is an example of "false flag" operations .

The US and allies had previously alleged that Russia would try to use one of these operations as a pretext for an invasion.

White House press secretary Jen Psaki said "we will continue to call out what we see as false flag operations or efforts to spread misinformation about what the actual status is on the ground."

What do we know about the rebels' request?

The two rebel leaders wrote to Putin to tell him that Ukrainian government shelling had caused civilian deaths, according to Peskov.

"The actions of the regime in Kiev are evidence of a refusal to end the war in Donbass," the letter said.

Separatist forces had initially said that they wanted to provide their own security.

The appeal also stated that the aggravation of the situation has forced many to leave their homes and evacuate to Russia, Interfax cited Peskov as saying. The two breakaway regions began calling for residents to evacuate to Russia late last week.

The heads of the DPR and LPR also accused Ukraine of destroying civil and industrial infrastructure of the two self-proclaimed "people's republics" in the statement cited by Interfax.

sdi, rc/ (AFP, AP, Interfax)