The UK has pledged to stand beside America in global efforts to “degrade and ultimately destroy” Islamic State militants fighting in Syria and Iraq, in a struggle that could last up to three years.
Nato has launched a a "core coalition" of 10 countries – led by the United States and including the UK, France, Germany, Canada and Australia – to tackle jihadists, amid speculation that Britain could soon join airstrikes.
The alliance's summit in Wales wrapped up with Cameron urging a "comprehensive plan" to deal with the growing threat, while refusing to rule out military intervention to defend our “national interest”.
But there were some signs of tensions, with UK officials concerned that the US is not putting enough emphasis on involving regional powers, and the French indicating they are not prepared to take part in military action in Syria.
IS militants have threatened to kill Scottish aid worker David Haines who they are believed to be holding in Syria. They have already beheaded two American journalists, posting the evidence on line in gruesome videos featuring a masked jihadist with a British accent.
US president Barack Obama insisted Nato members were "unanimous" in their commitment to stamp out to the "extremist nihilism" of IS - also known as ISIL and ISIS.
"We are going to achieve our goal. We are going to degrade and ultimately defeat ISIL, the same way that we have gone after al Qaida," he told a press conference.
At his own press conference, Cameron said: "My argument is you need that mixture of intelligent politics, diplomatic pressure, long-term engagement in a comprehensive plan as well as the potential for military or other more aggressive action.
"This will take time and it will take resolve. We will proceed carefully and methodically, drawing together the partners we need, above all in the region, to implement a comprehensive plan."
Despite evidence that Tory MPs are being canvassed over whether they would support involvement in airstrikes, Cameron said Britain was not yet at the stage where it was ready to take offensive military action.
He is thought to want to wait at least until a more inclusive Iraqi government is formed - which could happen next week.
Earlier, US secretary of state John Kerry appeared to try to up the tempo, calling for agreement on an international plan for dealing with IS by the time of the general meeting United Nations General Assembly in New York later this month.
Kerry and Foreign Secretary Philip Hammond jointly chaired a meeting of foreign and defence ministers from what the US called a "core coalition" of allies to discuss the way forward to eradicate the “genocidal, territorial-grabbing, caliphate-desiring” fighters.
Ministers from France, Australia, Germany, Canada, Turkey, Italy, Poland and Denmark were also at the talks.
"We have the technology, we have the know-how. What we need is obviously the willpower to make sure that we are steady and stay at this," Kerry said.
“We’re convinced in the days ahead we have the ability to destroy Isil. It may take a year, it may take two years, it may take three years. But we’re determined.”
There was some irritation among British officials at the apparent attempt to put a group of predominantly North American and European nations at the forefront of the effort at a time when they are seeking to build support from allies in the region.
Earlier Hammond said the Government will not be deterred from launching air strikes against IS by threats to the life of a British hostage they are holding.
"We will do everything we can to protect this individual (Mr Haines) but we have to be clear about the nature of the organisation that we are dealing with," Hammond told BBC News.
"They are utterly brutal, they are ideologically driven - almost pathological in the way that they behave. We cannot allow our strategy to be driven by their behaviour.
"We have to approach the challenge of Isil with focus purely on what is in Britain's best interests - protecting our citizens and our security from the threat that Isil poses from foreign fighters, from potential attacks on our soil, destabilisation of the region.
"We will not be diverted from doing what is right by the threats from this organisation."
Haines, 44, has a teenage daughter in Scotland from a previous marriage and a four-year-old daughter in Croatia from his present marriage.
Educated at Perth Academy secondary school, he has worked for aid agencies in some of the world's worst trouble spots, including Libya and South Sudan.
He was in Libya during its civil war in 2011, working as head of mission for Handicap International, which helps disabled people in poverty and conflict zones around the world.
Speaking to the Daily Telegraph at the family's home near Zagreb, his wife Dragana Prodanovic Haines, 44, said: "He's everything to us. He's our life. He's a fantastic man and father.
"Nobody can understand how we are feeling. My daughter keeps asking about him every day. She hasn't seen her father for a year and a half. She has gone through so much. She sees me crying all the time."