Egypt's military says it has bombed Islamic State (IS) targets in Libya, state television is reporting.

The strikes come after video emerged purporting to show 21 Coptic Christians being killed by militants identified as being from the Tripoli Province of IS.

Egypt's president, Abdel Fattah al-Sisi, has threatened to carry out a "suitable" punishment for the killings.

"Egypt reserves the right to respond in a suitable way and time to punish these murderers," he said.

The footage showed handcuffed hostages, which IS claims to have captured in Libya, in orange jumpsuits being beheaded by black-clad terrorists by the sea.

A caption on the five-minute video reads: "The people of the cross, followers of the hostile Egyptian church" and the producers identify themselves as the Tripoli Province of the Islamic State group.

Egypt's Copts are the largest Christian community in the Middle East and are estimated to account for around 10% of the Egyptian population.

The jihadists had said in the latest issue of their magazine that it held 21 Egyptian hostages.

Egyptian foreign ministry spokesman Badr Abdelatty confirmed that 20 Egyptians had been kidnapped in two incidents in neighbouring Libya.

He did not say when they were seized or specify their religious affiliation, but said seven Egyptians and 13 others abducted separately in Libya "are still being detained" by their captors.

Journalist Mary Fitzgerald, who has recently returned from Libya, told Newstalk Breakfast it is a significant development.

The beheadings come just days after IS released a video showing the gruesome burning alive of a Jordanian pilot it captured after his F-16 came down in Syria in December.

Earlier this week Newstalk's Shona Murray spoke to a former ISIS operative. You can listen back here.

Originally posted at 20.15