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 added: "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."