At least 35 more Egyptian Christians are feared to have been kidnapped by jihadists in retaliation for air strikes on targets in Libya.
Militants from the Islamic State and Ansar Al-Sharia are understood to have rounded up dozens of farm workers in the wake of bombings by Cairo, it was reported by local media.
The move is believed to be a direct response to strikes by Egyptian warplanes yesterday, which came after fanatics released a horrific video showing the beheading of 21 Christians on a beach.
It raises the chilling prospect of yet another mass execution in what is being seen as Isis's bid to announce its presence in a new region where it is gaining influence.
Initial reports said seven men had been seized, but that figure had risen to more than 35 by mid-afternoon yesterday, according to The Libya Herald.
It came as Egypt blitzed Isis training camps, weapons stockpiles and fighters in two waves of air strikes following the gruesome murder of captured Egyptian workers.
Meanwhile, the Egyptian government called for the US-led coalition to also target Isis in Libya.
The bombing, Egypt's first official military action in Libya since the overthrow of Muammar Gaddafi four years ago, came after a day of international outrage at the slickly produced ISIS video.
Lasting five minutes, it shows a group of Coptic Christians dressed in jumpsuits being marched one by one along a lonely beach, each held by a fighter clad in black.
The men their, faces uncovered, are then made to kneel.
A jihadist dressed in camouflage fatigues and speaking English with an American accent declares: "All crusaders... the sea you have hidden Sheikh Osama bin Laden's body in, we swear to Allah we will mix it with your blood."
The men are then forced to lie face-down before they are beheaded simultaneously. The camera zooms in on the water as it turns red.
The US said the video showed the "wanton killing of innocents".
Egyptian state TV showed warplanes over Libya as the military declared they were "avenging Egyptian blood and retaliating against criminals and killers".
Libyan officials said 40 fighters and two civilians had been killed.
The country's internationally recognised government, based in Tobruk, has been confined to the far east of the country since militias seized the capital Tripoli last year.
The militias condemned the air strikes as a violation of Libya's sovereign territory.
But Cairo called on the US-led coalition carrying out strikes over Syria and Iraq to broaden the scope of their operations to include Libya.
"Leaving things in Libya as they are without decisive intervention to suppress these terror groups constitutes a clear danger to international peace and security," the Egyptian foreign ministry warned.
French president François Hollande called for the UN security council to meet over Libya and to take new measures against the growing power of the militants.
Italy has already said it is willing to lead a coalition of European and North African countries to stop the ISIS advance in Libya. Significantly, it did not rule out using ground forces.
The video of the beheadings, posted online by Libyan jihadists loyal to IS, is the first to come from outside the group's heartland of Syria and Iraq, raising fears it has established a new foothold just 650km from Italy.
Pope Francis expressed "profound sadness" at the murders.
"They were executed for nothing more than the fact that they were Christians," he said.
"Their only words were: 'Jesus, help me!'"
Britain condemned the murders as "barbaric".
The Egyptian workers were seized in separate kidnappings in December and January from the coastal town of Sirte in eastern Libya, under the control of Islamist groups.
Libya, where wages are significantly higher than in Egypt, is home to a large community of both Muslim and Coptic Egyptians, with most working in the construction sector.
- Daily Mail