CAIRO — Libyan militants calling themselves part of the Islamic State said on Monday that they were holding captive 21 Egyptian Christians, raising new fears about the extremist group’s spreading influence beyond the battlegrounds of Syria and Iraq.

“Urgent — Soldiers of the Islamic State captured 21 Christian Crusaders in various areas of Tripoli Province,” declared a caption below three photographs of the hostages posted by the militants and first reported by the SITE Intelligence Group.

Last month, the same Islamic State outfit claimed responsibility for bombing an empty foreign ministry building in Tripoli in retaliation for a public wishing of “Merry Christmas” by a ministry official.

The statement came on the same day that hackers linking themselves to the Islamic State temporarily seized control of the Twitter account of the United States military’s Central Command, and just days after the emergence of a video-recorded vow of allegiance to the group by one of the perpetrators of last week’s terrorist attacks in Paris. There are now Islamic State affiliates in each of Libya’s three provinces. The group claiming to have seized the hostages identified itself as the Tripolitania Province of the Islamic State, named for the region around the capital, Tripoli, in western Libya.