His employer, Russia Today, said Philips, 35, was detained in Mariupol, Ukraine and is believed to have been taken for "interrogation" by the Security Service of Ukraine, known as SBU, the government security agency.
The Ukrainian National Guard said he has been detained and handed over to the SBU to transport to Kiev, after which he will be passed on to British embassy officials.
He has not been in contact with his employer since Tuesday afternoon, Russia Today said, adding an unidentified source informed the television channel Phillips would be released if he deleted certain video footage he took of a roadblock.
In his last Twitter message he said he had been "secretly filming" a Mariupol morgue.
"We still cannot get in touch with Graham. We are extremely worried what could happen to him," said Alexey Kuznetsov of Russia Today.