CAIRO (AFP) - Egyptian President Abdel Fattah al-Sisi said Egypt reserves the right to respond in a "suitable way" after militants in Libya released a video on Sunday showing the beheading of Coptic Christians.

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

The Islamic State in Iraq and Syria (ISIS) released a video on Sunday purportedly showing the beheading of Egyptian Coptic Christians they say they captured in Libya.

Mr Sisi immediately called an urgent meeting of the country's top security body and declared seven days of mourning. The Coptic Church issued a statement saying it was "confident" the killers would be brought to justice, while Al-Azhar, the prestigious Cairo-based seat of Islamic learning, denounced the "barbaric" killings.

The footage released by ISIS online shows handcuffed hostages wearing orange jumpsuits being beheaded by their black-suited captors in a coastal area the group said was in the Libyan province of Tripoli.

In the latest issue of the ISIS online magazine Dabiq, the group said 21 Egyptian hostages were being held in Libya, and in Sunday's video, shot from several angles, the beheadings of at least 10 hostages were seen.

Egyptian state television also broadcast some of the footage from the ISIS video.

The security body that will meet in Cairo groups Mr Sisi, his defence and interior ministers and top military figures.

Egypt last year denied reports of having carried out air strikes on Islamists in Libya.

"The Orthodox church... is confident its homeland would not rest until the evil perpetrators get their fair retribution for their wicked crime," the Coptic church said in a statement on its Facebook page.

In a statement, Al-Azhar said it had heard the news of the beheadings "of a group of innocent Egyptians with great sorrow and grief".

"Al-Azhar stresses that such barbaric action has nothing to do with any religion or human values."

The latest ISIS video comes just days after the militants released footage showing the gruesome burning alive of a Jordanian pilot the group captured after his F-16 came down in Syria in December.

The highly choreographed video showing the killing of Maaz al-Kassasbeh triggered global outrage.

Sunday's video, entitled "A message signed with blood to the nation of the cross", has a scrolling caption in the first few seconds referring to the hostages as "People of the cross, followers of the hostile Egyptian Church".

One of the masked captors, wearing a military uniform and pointing a knife to the camera said in English: "Today we are in the south of Rome, in the land of Islam Libya... the sea you have hidden Sheikh Osama bin Laden's body in, we swear to Allah we will mix it with your blood."

Al-Qaeda founder Osama was shot and killed in a dramatic helicopter raid by US special forces in Pakistan in the early hours of May 2, 2011 and later buried at sea in an unidentified location.

After the beheadings shown on Sunday, a scrolling caption on the footage said: "The filthy blood is just some of what awaits you, in revenge for Camilia and her sisters."

A background voice believed to be ISIS spokesman Abu Mohamed al-Adnani says: "We swear to Allah we will take revenge, even if it takes a while."

Egyptian women Camilia Shehata and Wafa Constantine were the wives of Coptic priests whose alleged conversion sparked a sectarian dispute in Egypt in 2010.

Shehata went missing for five days in July that year after a domestic argument before police found her and escorted her back home.

When she went missing, Coptic Christians staged protests, but when she was returned, Islamists took to the streets alleging she had willingly converted to Islam and was being held by the church against her will.

Wafa Constantine also went missing, in 2004, reportedly after her husband refused to give her a divorce. She was temporarily sequestered at a convent as reports of her conversion were circulated.

In January, the ISIS branch in Libya claimed it had abducted 21 Christians.

A spokesman for the Egyptian Foreign Ministry confirmed to AFP in Cairo at the time that 20 Egyptians had been kidnapped in two separate incidents.

Mr Badr Abdelatty 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.