A police statement named the two as Andrew Bagshaw and Christopher Parry and said in a statement they were trying to establish their whereabouts.

The police said the two left the city of Kramatorsk for the town of Soledar on Friday morning and were reported missing on Saturday evening after contact with them was lost. The police statement said they were aged 28 and 48 but gave no details of their voluntary work.

Kramatorsk is about 80 km (50 miles) from Soledar, where Ukraine's military says fighting is particularly intense.

"We are supporting the families of two British men who have gone missing in Ukraine," Britain's foreign office said.

(Reporting by Pavel Polityuk in Kyiv, additional reporting by Reuters in London, Editing by Timothy Heritage)