The self-proclaimed "Luhansk People's Republic" in the east of Ukraine has declared martial law, the republic's press office announced Thursday.

A full mobilization was declared for men between 28 and 45 years of age, according to Russia's ITAR-TASS agency.

Officials representing the separatists say the measure has come ahead of an expected attack by the Ukrainian National Guard, which is to storm the city of Luhansk later on Thursday, with around 200 combat units still on their way.

Apart from declaring martial law, the leadership has issued a warning that citizens might suffer a chemical attack "from the side of the National Guard and the Right Sector".

"People's Governor" in Luhansk, Valery Bolotov, meanwhile set out conditions to start talks with Kiev authorities if they put an end to what they describe as an "anti-terrorist" operation against rebels occupying public buildings.

Interfax agency has also quoted Bolotov as calling on Russian President Vladimir Putin to sent peacekeeping forces to the region.

In the neighboring region of Donetsk, clashes with separatists have taken the lives of Ukrainian soldiers, but the death toll could not be independently verified.

The latest development in Luhansk and Donetsk follow May 11's "self-rule" referendum held in the regions at which independence was reportedly approved by a huge majority.

Pro-Russian rebels have called for unification with Moscow and have said they would not take part in the presidential elections scheduled for May 25.