A British national has been detained in Ukraine, the Foreign Office has confirmed.
Graham Phillips is a journalist working for Russian TV station Russia Today.
The station said he had been detained in Mariupol, eastern Ukraine, on Tuesday. The National Guard later said it intended to transport him to Kiev.
The Foreign Office said it was "in contact with the Ukrainian authorities" and was ready to provide "consular assistance".
Mr Phillips, 35, who grew up in Nottingham before moving to Scotland, works as a freelance journalist and had been a stringer for Russia Today.In a statement, the organisation said he had not been in contact with anyone via phone, email or Twitter since 15:20 Moscow time on Tuesday.
It said he had been detained that morning and was believed to have been taken for "interrogation" by the Security Service of Ukraine, known as the SBU.
Ukraine's National Guard confirmed the journalist had been detained and passed to the SBU. It told the BBC he would be transported to Kiev and passed to British embassy officials, after which he should go free.
'Extremely worried' Russia Today said its repeated attempts to speak to representatives of the Ukrainian security service had been rebuffed.
It appealed to the British consulates in Russia and Ukraine, as well as to the UK Foreign Office, requesting their help with Mr Phillips's release.
Russia Today said he had called to tell them of his detention but they had since lost all contact with him.
The English language TV network claimed a source had told them Mr Phillips would be released if he deleted footage he had recorded near a roadblock.
Alexey Kuznetsov, deputy head of Russia Today's English department, said: "We still cannot get in touch with Graham, we are extremely worried what could happen to him."
On Monday Mr Phillips tweeted about growing calls for him to be arrested and deported from Ukraine.
His last tweet on Tuesday said that he had been "secretly filming" with a senior doctor at a morgue in Mariupol.
According to RT, during his last phone call with the station, Mr Phillips said he had been asked if he was a spy. He also told the station his car had been searched and his laptop confiscated.
The authorities checked his documentation, looked at his reports and clips, and asked him his position on Crimea, RT reported.
'Absolute rubbish' Meanwhile, MPs in the Russian parliament have expressed outrage following the detention of two Russian journalists.
Marat Saichenko and Oleg Sidyakin, from the pro-Kremlin news channel LifeNews, were detained by soldiers in eastern Ukraine.
They were accused of being armed and involved in "terrorism".
Speaking on Wednesday during his visit to China, President Vladimir Putin described the allegations against the reporters as "nonsense, absolute rubbish".
An online campaign is under way to secure their release - using images of people holding signs declaring "Save Our Guys".
There has been unrest in Ukraine as government forces continue to clash with pro-Russia separatists, who have taken over government buildings in southern and south-eastern parts of the country.
It comes after pro-Kremlin Ukrainian President Viktor Yanukovych was overthrown in February, following months of street protests.
A revolt in the east of Ukraine gained momentum after Russia annexed the country's mainly ethnic Russian region of Crimea in March.