Britain should join air strikes against the Islamic State of Iraq and the Levant (ISIL) in Syria, Prime Minister David Cameron has said.
Cameron attempted to persuade politicians on Thursday to back action in parliament, arguing the Paris attacks have given new urgency to the fight against ISIL.
He told the House of Commons that US President Barack Obama and French President Francois Hollande had urged Britain to join the military campaign.
"These are our closest allies and they want our help," he said.
Cameron said if Britain didn't act after ISIL-claimed attacks in Paris that killed 130 people, the UK's allies might well ask, "If not now, when?"
"We can't wait for a political transition. We have to hit these terrorists in their heartlands right now. And we must not shirk our responsibility for security or hand it to others," Cameron said.
"Throughout our history the United Kingdom has stood up to defend our values and our way of life. We can and we must do so again, and I commend this statement to the House," he added.
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Opposition leader Jeremy Corbyn fired back with a series of questions the Labour Party sought answers to.
"Without credible or acceptable ground forces, isn't the logic of an intensified air campaign mission creep and Western boots on the ground? Can he [Cameron] today rule out the deployment of British ground forces to Syria?" Corbyn asked.
"In the light of the record of Western military intervention in recent years, including Iraq, Afghanistan and Libya, does the prime minister accept that the UK bombing of Syria could risk more of what President Obama called 'unintended consequences' and that a lasting defeat of ISIL can only be secured by Syrians and their forces within the region?," the opposition leader further asked.
The Royal Air Force is part of a US-led coalition attacking ISIL in Iraq, but not in Syria.
Earlier this month, the parliament's Foreign Affairs Select Committee said British airstrikes would be "incoherent" and ineffective without a plan to end Syria's four-year civil war.
No 'safe haven'
Cameron replied on Thursday with a 36-page letter, arguing that Britain should act to deny the ISIL group a "safe haven" in Syria from which to plot mass-casualty attacks around the world.
He said air strikes should be part of a "comprehensive overall strategy" to destroy ISIL, end the Syrian war and help to rebuild the country.
Cameron wants to hold a vote in Parliament on air strikes, but has said he will only do so if he is confident he can win. He's likely to decide after Thursday's Commons debate whether to hold a vote next week.
Reporting outside parliament, Al Jazeera's Charlie Angela said thousands of air strikes had been launched against ISIL in recent months and yet the group still remained a force in the region.
"There's a big question whether Britain going in with air strikes would be all that effective," she said.
"This would be a good move diplomatically but it remains to be seen how effective militarily it would actually be."