British Prime Minister David Cameron warned that military action against Islamic State (IS) militants could last for "years" Friday as he urged lawmakers to back joining US-led air strikes in Iraq but not in Syria.

Kicking off a crunch debate in the House of Commons, Cameron said the "hallmarks" of the campaign would be "patience and persistence, not shock and awe".

"This is going to be a mission that will take not just months but years but I believe we have to be prepared for that commitment," he said, between a barrage of questions from lawmakers about the length and scope of the mission.

The debate has awakened memories in Britain of its role in the deeply unpopular US-led invasion of Iraq in 2003 under then prime minister Tony Blair.

But Cameron argued that the situation then was entirely different and the government has emphasised that lawmakers will not vote on sending combat troops.

"This is not 2003 but we must not use past mistakes as an excuse for indifference or inaction," he added.

Six British Tornado fighter jets based in Cyprus are poised to begin raids on IS within days or even hours if the vote, due at 5:00 pm (1600 GMT), is passed.

Britain would join the US and France in launching targeted strikes on the IS group in Iraq, where it controls swathes of territory, as in neighbouring Syria.

IS fighters have beheaded a British aid worker, David Haines and two US journalists, and are holding two other Britons, Alan Henning and John Cantlie.

Britain does not propose, as yet, joining US-led air strikes on Syria, which are backed by five Arab states. Cameron said a separate parliamentary vote would be needed for that to happen.

He admitted there was "no consensus" on action in Syria, although he said that "Britain should do more" there.

Some lawmakers, for whom the 2003 invasion remains a painful memory, are expected to oppose military action because of fears that the mission is ill-defined.

Between 2003 and 2009, 179 British personnel died in Iraq and the last British troops left the country only in 2011.

"I believe there's a big danger of mission creep with this and no-one can tell us what the endgame is," Labour lawmaker Diane Abbott told Sky News before the debate.

Blair, prime minister when Britain went to war in Afghanistan in 2001 and Iraq in 2003, has urged Cameron not to rule out sending ground troops again "if it is absolutely necessary".

The Stop the War Coalition -- which helped organise a million-strong demonstration against the 2003 Iraq war -- staged a protest of around 200 people Thursday and has vowed further demonstrations if the vote passes.

- Hostage safety concerns -

Mindful of a war-weary public and a damaging parliamentary defeat last year over military action against President Bashar al-Assad's regime in Syria, Cameron's government has prepared the ground carefully for Friday's vote.

The parliamentary motion, released in advance, stresses that the government will not deploy ground forces and that Iraq's government had requested assistance to fight the IS group, making it legal under international law.

Cameron has also argued for weeks that the "murderous plans" of extremists pose a direct threat to British people.

On Thursday, police in London arrested nine people suspected of extremist Islamist links and another two were held Friday.

Officials believe 500 Britons have travelled to fight with IS and may include a militant with a British accent who is thought to have killed Haines and two US journalists.

The family of Henning, a taxi driver captured while delivering aid in Syria, has voiced fears that air strikes against IS group targets could put his safe return in jeopardy.

"If they're going to do air strikes on them, they'll just run away. They'll take him with them and no-one will know where he is again," his brother-in-law Colin Livesey told ITV television this week.