Foreign Secretary Philip Hammond said he would not enter into a discussion with Bashar al Assad's government, after the former head of the army Lord Dannatt argued Britain and Syria should co-operate to defeat a common enemy in the Islamic State (IS).
The peer, whose position was backed by senior Tory Sir Malcolm Rifkind, argued "a conversation has got to be held" between the UK and Assad, following Tuesday's beheading of American journalist James Foley by a jihadist who apparently had a British accent.
"If there are going to be any questions of air strikes over Syria airspace it's got to be with the Assad regime's approval," Lord Dannatt said.
However Mr Hammond said that an alliance with the dictator, who the UK Government has said should be removed from power for his actions during the Syrian civil war, would prove "poisonous".
"I have said very often that one of the first things you learn is that my enemy's enemy is not my friend," Mr Hammond said yesterday.
"We may find we are aligned against a common enemy but it would poison what we're trying to achieve in separating moderate Sunni opinion from the poisonous ideology of Isil [Islamic State of Iraq and the Levant] if we were to align ourselves with Assad."
IS have made sweeping gains in northern Iraq, leading the United States to declare the group the biggest threat it has faced in recent years.
Efforts are ongoing to identify the masked man responsible for Mr Foley's murder, who has been named as a London-based extremist called John in unconfirmed reports. IS had threatened to kill another American if the US did not halt airstrikes against the group.
Meanwhile, in another day of turmoil in Iraq, militants attacked a Sunni mosque in a volatile province outside Baghdad during Friday prayers yesterday, killing at least 64 people. An army officer and a police officer said the attack on the Musab bin Omair Mosque in Imam Wais village, 75 miles north-east of Baghdad, involved a suicide bomb and gunmen who stormed inside the mosque and fired on worshippers.
It was not immediately clear if the attack was carried out by Shiite militiamen or IS, however, the atrocity prompted Sunni lawmakers to pull out of talks on forming a new Iraqi government. The development is a major challenge for prime minister-designate Haider al-Abadi, a Shiite who is struggling to form an inclusive government that can confront the militants.
The two major parliamentary blocs, who ended talks, pointed the finger at Shiite militias and demanded that outgoing prime minister Nouri al-Maliki and the main Shiite parliamentary bloc hand over the perpetrators within 48 hours and compensate victims' families "if they want the political process and the new government to see the light of day".
l An alleged extremist has appeared in court accused of bankrolling jihadists in Syria. Asim Ali, 33, from Southall, west London, appeared at the Old Bailey via video link. Mr Ali, who was remanded in custody, is charged with assisting Imran Khawaja and others to commit acts of terrorism.