A Russian soldier has shot dead a Ukrainian naval officer in eastern Crimea, Ukraine’s defence ministry has claimed, in one of the few fatalities reported since Russia took control of the Black Sea peninsula.

Russian media reports said a group of Ukrainian soldiers in the village of Novofedorivka had been drinking and were on their way home when they passed Russian soldiers guarding an entry to the military base where they previously worked, and an argument broke out between the two groups.

“We confirm the death of a Ukrainian officer in the village of Novofedorivka in Crimea,” the ministry’s press service told Interfax-Ukraine. According to the news agency, the Russian serviceman shot dead the Ukrainian officer near the hostel where the latter lived.

Dmytro Tymchuk, the director of the Centre for Military and Political Studies, said on Facebook on Monday that Russian junior sergeant Ye S Zaitsevv shot a Ukrainian serviceman – who he named as Major K of military unit No 1100 – twice in the head at point-blank range as he was making his way back to his hotel.

He added that Captain A Yermolenko, who was sharing a hostel room with the murdered Ukranian, had been beaten and abducted by Russian troops. Tymchuk said that Russian servicemen had also taken the body of the Ukrainian major.

A criminal investigation has been opened.

News of the death came as Ukraine’s prime minister accused Russia of sowing unrest in his country’s eastern provinces as a pretext for dispatching troops across the border.

Speaking at an emergency cabinet meeting on Monday, Arseniy Yatsenyuk said Russia was behind the seizure of several government buildings in eastern regions that had led to an increase in secessionist sentiment.

“The plan is to destabilise the situation, the plan is for foreign troops to cross the border and seize the country’s territory, which we will not allow,” he said, adding that people engaged in the unrest had distinct Russian accents.

Yatsenyuk said Russian troops remained stationed within 19 miles (30km) of the frontier.

Earlier in the day, the interior ministry reported that armed gunmen had occupied a security services building in Luhansk, 15 miles west of Russia and scene of frequent protests since the country’s pro-Moscow president was ousted in February.

Police in Luhansk say they have been put on alert and have blocked all entrances to the city.

A crowd of pro-Russian activists stormed the building on Sunday. Local media reported that demonstrators pelted the building with eggs, and then stones, a smoke grenade and finally a firebomb. The flames were reportedly quickly extinguished. Police said nine people were injured during the assault on the building.

Eastern Ukraine was the heartland of support for Viktor Yanukovych, the president who fled to Russia in February after months of protests. About half of the region’s residents are ethnic Russians, many of whom believe Ukraine’s acting authorities are Ukrainian nationalists who will oppress Russians.

Ukraine’s interim authorities deny they are infringing the rights of the ethnic Russian population. Russia has moved large contingents of troops near the Ukrainian border, amid speculation that unrest in eastern Ukraine could be used as a pretext for a Russian incursion.

Since Crimea held a secession referendum and was annexed by Russia in March, calls for similar votes in Ukraine’s east have emerged.

Also on Sunday, crowds assaulted provincial government buildings in the eastern cities of Donetsk and Kharkiv.

The interior minister, Arsen Avakov, said on Monday that the Kharkiv building had been retaken by the authorities. In Donetsk, local media reported that groups occupying the provincial government building have barricaded the entrance with car tyres and barbed wire.