Ukrainians are being arbitrarily detained and subjected to enforced disappearances in Russian-controlled areas, the UN has told the BBC.

At least 36 cases of civilian detentions were verified by the UN, with families often denied any information about the fate of those being held.

Ukrainians say they fear an escalating campaign of kidnappings and intimidation, as Russia struggles to assert control over towns it captures.

Viktoriia Roshchyna, a journalist, was working in occupied areas in the east of the country when she was taken by unidentified men on 15 March.

Her employer, Hromadske media, said she "was probably detained by the FSB", Russia's internal intelligence service, based on witness accounts of her being taken in the city of Berdyansk.

She was released six days later when a hostage-style video - apparently recorded under duress - began to circulate on pro-Russian Telegram outlets. In it Ms Roshchyna said Russia had not taken her captive and thanked Moscow's forces for "saving her life".

Svetlana Zalizetskaya, a journalist in the occupied city of Melitopol, accused Russian forces of taking her 75-year old father hostage as punishment for her refusal to co-operate with the new administration.

Ms Zalizetskaya, the director of local news agency RIA Melitopol, wrote on Facebook that her father had been detained after her meeting with the Russian-installed leader of the city, where she refused to end her criticism of the invasion.

She said that she received a phone call from his captors, in which her father informed her that he was being held "in some basement" and said that he "didn't know what they wanted from him".

His captors demanded that Ms Zalizetskaya, who has pledged to "tell the world of atrocities" committed by Moscow's forces in Melitopol, surrender herself.

Ukraine's National Union of Journalists (NUJ) said four journalists had also been detained and were later released in Melitopol.

The head of the Ukrainian NUJ, Sergiy Tomilenko, said the detentions were part of "a wave of information cleansing" which is aimed towards the "intimidation of journalists and public figures".

A spokesperson for the UN's Office of the High Commissioner on Human Rights (UNOHR), whose monitoring mission in Ukraine has been documenting the abductions, told the BBC that those being targeted "are mostly representatives of local communities, journalists and people who were vocal about their pro-Ukrainian positions".

But they said they were not able to assess whether those being detained form part of "targeted lists reportedly drawn up by Russian security officials".

In February, US officials sent a letter to the UN warning that Russia had drawn up a "kill list" of Ukrainians to be attacked or detained following the invasion of the country.

Some sources have suggested that a hack of a Ukrainian digital services app in January may have helped Russia identify targets, while the Royal United Services Institute (RUSI) has speculated that a hack of the Ukrainian national car insurance registry may have allowed security services to identify the locations of targets.

A number of officials in occupied parts of Ukraine have been detained by Russian forces in recent weeks. In the southern city of Melitopol, Mayor Ivan Fedorov was abducted from a city crisis centre by Russian troops earlier this month.

Discussing his detention with local media, Mr Fedorov alleged that other detainees at the holding centre he was taken to were being tortured.

"They didn't touch me physically but trust me, seven armed men were enough to make their position clear," Mr Fedorov said. "In the next cell someone was being tortured - there were screams which generated plenty of psychological pressure."

"They try to accuse them of sabotage and squeeze their fingers in the door to make them say which army they're from, but they're just local residents."

Alleged abductions have occurred in several other cities, including Nova Kakhovka in the north, where the secretary of the city council has disappeared, and in Bucha, where the local council told the BBC that six employees were detained and later released after a Russian raid.

Ukrainian MP Alyona Shkrum said she believed the detentions were likely to increase as Russia faces a lack of co-operation and increased resistance in occupied regions.

"I'm sure Putin thought it will be just as it was in Crimea, they come they take over the administrative buildings and the mayor will say 'lets co-operate, I will be your mayor now, what difference does it make'," she told the BBC.

"It didn't happen at all here... nobody, even from sort of pro-Russian parties agreed to do what Russian soldiers wanted them to do".

Ms Shkrum, a pro-Western MP, told the BBC that she was warned by the Ukrainian security services that she was likely on a Russian hit-list and was warned to avoid her apartment in Kyiv.

"Pretty much there are two lists," she said. "A list of people to be killed as members of parliament, they are mostly people who [Russia] feel they cannot co-operate with. And a list of people to be taken hostage and taken to Moscow and forced to vote on something."

"I also have sanctions against me in Russia, so I presume I am on the list to be killed or captured," Ms Shkrum added.

The abductions have not been limited to public figures, with some civilians and ex-members of the armed forces also detained.

Mattia Nelles, a Ukrainian political analyst, told the BBC that his uncle, a former medic with the Ukrainian army who fought in the Donbas from 2016-2018, had been targeted by Russian soldiers in the eastern city of Svatove.

"Security forces of the Russian Federation have come to his house and searched for him," Mr Nelles said. "Luckily he was not at his house but neighbours confirmed that he is wanted."

Mr Nelles said it was unclear which element of Russia's forces are behind the wave of detentions, but in his uncle's case he said the troops were "a mix of army personnel and some other security forces. We can only speculate whether it's FSB, they didn't wear insignia".

He fears that the campaign of arrests will escalate in the coming days. He observed that in southern areas, such as Kherson, so far the "amount of arrests seem relatively low" and some of the lists appear to be "drawn up on the go" as Russian forces identify those resisting occupation.

"Locals fear we are seeing the beginning of a more sustained campaign," he warned.