Ukrainian forces have exchanged gunfire with unknown armed men in the first reported gun battle in the east of the country, where pro-Russian protesters have seized a number of government buildings in recent days.

At least one person was killed and several others were wounded in Slaviansk. Ukraine's interior minister, Arsen Avakov, said on Facebook there had been casualties on both sides, and that a Ukrainian officer had died.

A video seen by the Guardian showed the aftermath of the attack, including a wounded Ukrainian soldier and what appeared to be a dead man in a black uniform with a machine gun, thought to be one of the militia.

Vladimir Kolodchenko, a council member in the nearby city of Nikolayevka who witnessed the gun battle, said a convoy of seven armoured troop carriers had arrived in Slaviansk to try to end the occupation of the buildings. He said the commanders had gone to negotiate with the protesters and that the convoy had come under attack in their absence.

After the firefight, the unknown men fled, leaving their car at the scene. The troops also later withdrew.

Violence was also been reported at protests in Kharkiv, and pro-Russian forces were said to have occupied official buildings at several towns in the east of Ukraine. Occupations were reported to be taking place in Donetsk, Kramatorsk, Druzhkovka, Mariupol and Yenakievo.

Donetsk was the stronghold of the former president Viktor Yanukovych, who fled after months of pro-European protests in Kiev.

The situation remained tense in Slaviansk on Sunday. Outside an occupied police station, masked men with machine guns and handguns warmed themselves by fires while others manned barricades on either end of the street. Protesters chanted "Russia" and "Putin, help".

A Russian Orthodox priest named Father Nikolai climbed on to the barricade and called for unity. He said: "Pray for peace. Don't just chant but pray … The church has always been and will always be on the side of the people."

In the video of the gun battle between troops loyal to Kiev and Moscow shot by Kolodchenko immediately after the attack, a soldier is shown lying on the ground in obvious pain, holding his side. Two armoured troop carriers are visible on the road, although Kolodchenko said there were initially seven.

A man in a black uniform is on the ground leaning against another vehicle, his head slumped over his chest, blood running down his front. A machine gun lies by his side.

"I checked his pulse with my hand and he was absolutely cold," Kolodchenko said.

He believed the men had attacked in an attempt to provoke the soldiers into a reaction.

Avakov had described the unrest as "Russian aggression". In an earlier post on Facebook he called on residents of Slaviansk to remain calm and stay at home.

In a phone call with the Russian foreign minister, Sergei Lavrov, the US secretary of state, John Kerry, expressed concern that the attacks "were orchestrated and synchronised, similar to previous attacks in eastern Ukraine and Crimea", according to the US state department. Kerry "made clear that if Russia didn't take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences", the department said.

The Russian foreign ministry denied Kerry's claims, while Lavrov blamed the crisis in Ukraine on the failure of the Ukrainian government "to take into account the legitimate needs and interests of the Russian and Russian-speaking population", the ministry said. Lavrov also warned that Russia may pull out of next week's Ukraine summit if Kiev used force against "residents of the south-east who were driven to despair".

The secretary general of Nato, Anders Fogh Rasmussen, said on Sunday that he was concerned by rising tensions in eastern Ukraine, which he described as "a concerted campaign of violence by pro-Russian separatists, aiming to destabilise Ukraine as a sovereign state". He likened the developments in the east to what happened in Crimea last month, when men in unmarked uniforms occupied the region.

"Any further Russian military interference, under any pretext, will only deepen Russia's international isolation," he said.

The Swedish foreign minister, Carl Bildt, who was in Ukraine this weekend, condemned the unrest in a Twitter post as "a co-ordinated armed action to seize control over key parts of eastern Ukraine", which "would not have happened without Russia".

In Slaviansk, the mayor said on Saturday that the men who seized the police station were demanding a referendum on autonomy and possible annexation by Russia. Protesters in other eastern cities have made similar demands after a referendum in Crimea last month in which voters in the autonomous region opted to split from Ukraine, leading to annexation by Russia.

Overnight, the interior minister reported an attack on a police station in the nearby city of Kramatorsk. A video from a local news website Kramatorsk.info showed a group of camouflaged men armed with automatic weapons storming the building. The website also reported that supporters of the self-declared separatist Donetsk People's Republic had occupied the administration building, built a barricade with tyres and flown a Russian flag nearby.

A regional news website, OstroV, said three key administrative buildings had been seized in another city in the area, Enakiyeve. In Mariupol, a city south of Donetesk on the Azov Sea and just 30 miles (50km) away from the Russian border, the town hall was seized by armed masked men. A local news website 0629.com.ua said about 1,000 protesters were building a barricade while unknown armed men raised the Russian flag over the building.

On Saturday in Donetsk, the regional capital, witnesses said the men who entered the police building were wearing the uniforms of the Berkut, the feared riot police squad that was disbanded in February after Yanukovych's ousting. Berkut officers' violent dispersal of a demonstration in Kiev in November set off the mass protests that culminated in bloodshed in February when more than 100 people died in sniper fire. The acting government says the snipers were police officers.