David Cameron condemns 'barbaric' IS beheadings in Libya
Libya must not become "a safe haven for terrorists" after the apparent beheadings of 21 Egyptian Christians in the country, David Cameron has said.
The UK prime minister condemned the "barbaric" killings after Islamic State posted a video online.
He said the UK must remain "steadfast" in efforts to defeat IS and bring a political transition in Libya through the UN.
The Archbishop of Canterbury described the killings as a "terrible cruelty".
Mr Cameron said the UK stood "united with the Egyptian people" and "our efforts to defeat the monstrosity of Islamist extremism must not waver."
A Downing Street spokesman said Mr Cameron had spoken to Egyptian president Abdel Fattah al-Sisi and both agreed the crisis in Libya must be resolved in the long term "through a comprehensive political solution".
The spokesman added: "The PM emphasised the importance of the UN-led political process in Libya producing a national government to lead the fight against terrorism through strong institutions."
Egypt has launched air strikes on positions held by IS in Libya after the emergence on Sunday of a video showing the 21 Coptic Christians, who were captured in December and January from the coastal town of Sirte, in Libya, being forced to the ground before apparently being beheaded.
The killings have raised fears that IS could establish a firm presence in Libya, which has had no unitary government since Muammar Gaddafi, the country's long-standing ruler, was overthrown in 2011.
In 2011 the UK and the US fired missiles at Gaddafi's forces to enforce a UN-mandated no-fly zone to protect civilians in the city of Benghazi, where rebels had seized control.
Asked if Mr Cameron still believed it was a good idea to have intervened in Libya in 2011, the prime minister's official spokesman said: "This was a country where people were being oppressed by a dictator, where they were not able to pursue their aspirations and have their voice heard for a democratic and peaceful Libya.
"The actions we took there and the decision to intervene was an international one."
The Egyptian government has called on the international community to intervene against IS militants in Libya following the killings.
Meanwhile, the Egyptian Ambassador to the UK, Nasser Kamel, told the BBC that Libya was a problem for Europe in particular because of its closeness to Italy and that there would be "boats full of terrorists" if the international community did not act together.
However, when asked whether Britain might provide military support for Cairo, Mr Cameron's spokesman said: "Our focus is on the political solution and there are no discussions beyond that at the moment in government."
The UN has called for an end to conflict in Libya, where rival armed groups have been fighting for power, and has previously brokered talks between rival parliamentary factions.
Foreign Secretary Philip Hammond said acts of terrorism should not "undermine" Libya's political transition towards democracy and the rule of law.
Mr Hammond said: "We remain fully supportive of the UN's efforts to build a national unity government for Libya and to bring a political solution to the ongoing security crisis.
"Those who support terrorists can have no part in this process."
The Archbishop of Canterbury, Justin Welby, the leader of the Church of England, spoke of his "deep compassion for the bereaved and killed" in Libya and also commented on the recent shootings in Denmark and a suicide bombing in Nigeria.
"The killers seem to rejoice in ever more extreme acts carried out to inflict greater terror," he said.
The archbishop said he was praying "for governments affected to be wise and courageous."
Michael Nazir-Ali, a Pakistani-born British Anglican bishop and former Bishop of Rochester, said there needed to be "restoration of order" in the country, which "cannot wait for a political solution".
Bishop Nazir-Ali told BBC Radio 4's World at One programme: "If, under UN auspices, there is some authorisation for a force to restore order in Libya, that would be welcome by all sorts of people.
"That does involve, I'm sorry to say, ground forces."
Bishop Nazir-Ali added: "We are dealing with a situation where we have to ask, 'Who is the lesser monster?'
"Take Syria, for instance. President Assad is no saint but compared to what IS is doing, we have to ask should we deal with this man to get rid of this much greater evil?"
The leader of the Coptic Orthodox Church in the UK, Bishop Angaelos, said the brutality was "plunging the world back into a medieval time".
The bishop said it was time to "quash" edicts that used religion to justify the taking of life, and that the only solution was to appeal to those who have an "element of humanity" to turn away from the barbarism.
Bishop Angaelos said he gained no joy or comfort from the idea that IS members may have been killed in the Egyptian air strikes.
"Nations have a right to rule in the way they see fit, but war and killing is so destructive and so wasteful of life and should really be avoided.
"I think the only solution is to keep appealing to humanity."
Sir Richard Dalton, a former British ambassador to Libya, said IS has established a presence in the country following the "disappearance of legitimate authority".
However, he stressed the scale of IS's influence appears to be "limited" and played down the possibility of the militant group building a power base similar to those it established in Syria and Iraq.
Mr Cameron discussed action against Islamist extremists with the Sultan of Brunei during talks on Monday at the prime minister's country residence, Chequers, in Buckinghamshire.
Number 10 said afterwards: "They agreed on the importance of making sure moderate Islam prevailed over extremist ideology and how both countries could continue to work to support this both at home and abroad."