Egypt's military said in a statement on state television it had carried out an air strike against Islamic State targets in Libya at dawn on Monday, a day after the group released a video appearing to show the beheading of 21 Egyptians there.

The attack focused on Islamic State camps, training sites and weapons storage areas in Libya, where Islamist militants have thrived amid chaos.

President Abdel Fattah Al-Sisi has repeatedly said militants based in Libya pose a serious security threat to Egypt, a strategic US ally that is fighting insurgents in the Sinai who have pledged allegiance to Islamic State.

The 21 Egyptian Christians, who had gone to Libya in search of jobs, were marched to a beach, forced to kneel and then beheaded, the video showed.

Speaking on national television hours after the release of the video, Sisi said Cairo would choose the "necessary means and timing to avenge the criminal killings".

Egypt's state news agency Mena quoted the spokesman for the Coptic Church as confirming that 21 Egyptian Christians believed to be held by Islamic State were dead.

The beheadings could stiffen Sisi's resolve in dealing with security threats from militants thriving in neighbouring Libya's chaos who want to topple his US-backed government.

Egypt has denied reports in the past that it had taken part, along with its close ally the UAE, in air strikes against militants based in Libya.

The footage showing the deaths of the Egyptians appeared on the Twitter feed of a website that supports Islamic State, which has seized parts of Iraq and Syria and has also beheaded Western hostages.

In the video, militants in black marched the captives, dressed in orange jump suits, to a beach the group said was near Tripoli. They were forced down onto their knees, then beheaded.

A caption on the five-minute video read: "The people of the cross, followers of the hostile Egyptian church." Before the killings, one of the militants stood with a knife in his hand and said: "Safety for you crusaders is something you can only wish for."

Thousands of Egyptians desperate for work have travelled to Libya since an uprising at home in 2011, despite advice from their government not to go to a country sliding into lawlessness.

Sisi, who met with the country's top military commanders to discuss the killings, called for a seven-day mourning period, state television reported. - Reuters