Britain has ruled out any alliance with Syrian president Bashar Assad to combat the threat posed by Islamic State (IS) extremists.
The Government has come under pressure to contemplate working with the Assad regime to tackle the militants operating in Syria and Iraq, with former head of the army Lord Dannatt suggesting there was a need to build bridges with the Syrian president.
But Foreign Secretary Philip Hammond said an alliance with the Assad regime would not be "practical, sensible or helpful".
The UK government has called for Assad to be removed as Syrian leader as a result of his actions during the country's bloody civil war.
Asked if the UK would have to collaborate with the Assad regime, Mr Hammond told BBC Radio 4's World at One: "No. We may very well find that we are fighting, on some occasions, the same people that he is but that doesn't make us his ally."
Former chief of the general staff Lord Dannatt told the BBC: " The old saying my 'enemy's enemy is my friend' has begun to have some resonance with our relationship with Iran.
"I think it's going to have to have some resonance with our relationship with Assad."
And former foreign secretary Sir Malcolm Rifkind suggested that the West would have to deal with Assad in the same way that Sir Winston Churchill allied with Joseph Stalin against Hitler.
Sir Malcolm told World at One: "I think we have to be harshly realistic, which means we don't pretend we are chums of the Syrian regime - they are a ghastly regime, they are a horrid regime - but just as during the Second World War Churchill and (Franklin D.) Roosevelt swallowed hard and dealt with Stalin, with the Soviet Union, not because they had any naivety about what Stalin represented but because that was necessary in order to defeat Hitler, and history judged them right in coming to that difficult but necessary judgment."
But Mr Hammond, who refused to rule out arming Syrian rebels to help them fight IS, said an international alliance rather than a collaboration with Assad was necessary.
"We are not going to do this on our own, except in terms of our strictly domestic security situation in the UK, we are going to do it as part of an international coalition," he said.
"We are going to do it as part of an international coalition led by the United States working with the Iraqis, because the problem has to be tackled first of all in Iraq where Isil (IS) has made its recent gains.
"We have a clear strategy that is based on a security track and a political track as well as a humanitarian track.
"There is a new government in the process of being formed in Baghdad which we hope and expect will be a more inclusive government that offers something for the Sunni population of Iraq that is different, distinctive and more attractive than what Isil are offering.
"We hope that once that government is formed and operating we will then be able to get behind the Iraqi government, the Iraqi security forces, and help the Iraqi state to push back against this insurgent organisation."
Mr Hammond said: "I don't know where the idea comes from that Assad has to assent to a military intervention in his country. There is a civil war raging...
"There are all sorts of practical issues. But I do not think that engaging in a dialogue with the Assad regime would advance the cause that we are all advocating here.
"One of the first things you learn in the Middle East is that my enemy's enemy is not my friend.
"We may very well find that we are aligned against a common enemy. But that does not make us friends with someone, it doesn't make us able to trust them, it doesn't make us able to work with them.
"It would poison what we are trying to achieve in separating moderate Sunni opinion from the poisonous ideology of IS if we were to align ourselves with President Assad."
Mr Hammond repeated that Britain would not be "putting combat boots on the ground", insisting: " This is not a fight that can be won by Western military force on the ground... t his needs to be a fight that is dealt with by Iraqis on the ground."
He suggested the UK could supply weapons and training to Kurds and Iraqi forces once there is a "credible Iraqi government that represented all the people of Iraq".
Although Mr Hammond stressed that the Government's policy was to provide only non-lethal support to Syrian opposition, he refused to rule out a shift.
"We review our position with regard to the Syrian moderate opposition periodically," he said. "The last time we did so we concluded that it was not appropriate at that time to consider moving from our position of providing non-lethal aid to move to one of providing lethal aid.
"We will go on reviewing that periodically. But at the moment our judgement is that we should not provide lethal aid."
On the hunt for the extremist known as "John" who carried out the on-screen beheading of American journalist James Foley, Mr Hammond said: "Our intelligence agencies and police are studying the material that we have received.
"You wouldn't expect me to give a running commentary on that process, but we are devoting significant amounts of resource to identifying the individual."
Mr Hammond insisted that robust policing and effective intelligence meant the Government was "well positioned" to manage the problem of UK extremists who travelled to Syria and Iraq.
"We have a significant number of powers available to us to deal with people in the UK planning to travel abroad to take part in terrorist-related activities," he said.
"We can withdraw passports; in some cases people with dual nationality can have their British nationality withdrawn. We have the ability to monitor them while they are outside the UK and they can be arrested on their return to the UK and prosecuted for acts they carried out outside the UK."
Former Foreign Office minister Kim Howells, who also chaired the Intelligence and Security Committee, said the UK was paying a "bitter price" for failed multiculturalism policies.
Describing a "narrative" that all Muslims were the victim of a plot to suppress them, the Labour politician told the World at One: "I'm afraid that the imams, the religious leaders within those communities, have never done enough to counter it, let alone governments.
"Governments have been afraid to touch it, they have allowed those communities to become isolated, to look inward, instead of trying to integrate with the rest of society and they have said 'well, that's all right, that's multiculturalism'.
"Well I'm afraid that we're paying a very, very bitter price for that now."
But Labour MP Yasmin Qureshi told the programme that Mr Howells' comments were "simplistic and completely unrealistic".
She added: "A lot of these young people, if they are being radicalised, it's being done through the internet and it's being done through social media."
Ms Qureshi said many young Muslims highlighted "inconsistencies and hypocrisies" over the West's response to the crisis in Gaza and the situation with IS in Iraq.
"Just as everybody now seems to be saying the narrative that sometimes these people may be, wrongly, motivated by what they see out there world events-wise is wrong, we also need to look at our international policy to actually counteract some of their thinking," she added.