Ukrainian authorities have issued an ultimatum to pro-Russian protesters in the country’s eastern regions to vacate the government offices they seized on Sunday or be dislodged by force.

Acting Interior Minister Arsen Avakov warned on Wednesday that the standoff in Donetsk, Lugansk and Kharkiv will be resolved within the next 48 hours one way or the other.

"There are two ways to end the crisis - political settlement and use of force. We are ready for both scenarios," Mr Avakov told reporters in Kiev.

Ukrainian TV channel 1+1 reported on Wednesday that armoured personnel carriers had been rushed to Lugansk where armed protesters had barricaded themselves inside the security service headquarters.

Protesters in Lugansk and Donetsk have reinforced the barricades set up near the occupied government buildings.

On Monday, the protesters in Donetsk and Kharkiv proclaimed independence from Kiev and called for a referendum on joining Russia.

Ukrainian lawmaker Serhiy Tihipko, who met with protesters in Lugansk, said the government had provoked the protests by ignoring people’s concerns.

"The authorities are not heeding the voice of the southeast. People want changes but their demands are ignored and they are branded separatists," Mr Tihipko, who is an independent presidential candidate in the May 25 elections, said after a visit to Lugansk.

Russia has denied any intention of using the standoff in eastern Ukraine for invading the country as alleged in Kiev and Washington.

"The United States and Ukraine have no reasons for concerns," the Russian Foreign Ministry said on Wednesday. "Russia has said many times that it is not conducting any unusual or unplanned activities significant in terms of military issues on its territory in the area of the Ukrainian border."

‘Strange U.S. response’

Meanwhile, Moscow said it had received a «strange» denial definitive response from the U.S. about reported deployment in Ukraine’s eastern regions of 150 American mercenaries from the Greystone private security service.

"They said they saw no basis for this to be true," Russian Foreign Minister Sergei Lavrov complained on Wednesday. "It’s a strange reply. Why not just say they are not there."

Mr. Lavrov said he would attend a proposed meeting with the Foreign Ministers of Ukraine, the European Union and the United States in Brussels next week provided its agenda would include a constitutional reform in Ukraine.

The four-party talks were announced in Brussels following Mr Lavrov’s telephone talk with E.U. foreign policy chief Catherine Ashton on Tuesday.

"The crisis in Ukraine can only be resolved if there is a equal, respectful dialogue involving all regions," the Russian Foreign Minister said on Wednesday.

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