Britain looks set to ramp up its military involvement in the struggle against the Islamic State terror group, as David Cameron addresses the United Nations in New York.

Ahead of the address, Mr Cameron is likely to receive a formal Iraqi request for UK involvement in air strikes when he meets prime minister Haider Abadi, head of the new inclusive administration in Baghdad.

And he will become the first British Prime Minister since the Islamic Revolution 35 years ago to hold face-to-face talks with an Iranian president, as he seeks to enlist Hassan Rouhani into the international coalition being assembled by US president Barack Obama to tackle the Sunni extremists.

Mr Cameron yesterday laid out a stark warning that IS - also known as Isis or Isil - is planning terror attacks in Britain, Europe and the US, saying: "This is a fight you cannot opt out of. These people want to kill us. They've got us in their sights."

Speaking to US TV channel NBC, the PM said there was "no doubt in my mind" that IS was to blame for the murder of four people at a Jewish Museum in Brussels in May and was planning further outrages in the West.

"There are other plots they have been attempting, including in my own country, in order to kill and maim innocent people," said the PM. "And the same applies to the United States of America.

"So this is a fight you cannot opt out of. These people want to kill us. They've got us in their sights and we have to put together this coalition ... to make sure that we ultimately destroy this evil organisation."

Iraq has not yet formally asked Britain to join the US and France in air strikes on IS forces which have seized control of large swathes of northern Iraq, though the UK responded swiftly to Baghdad's plea for heavy machine guns and ammunition earlier this month and Downing Street aides indicated that Mr Cameron was ready to consider further requests from Mr Abadi when they meet for the first time at the UN building.

A request from the Baghdad government would provide legal underpinning for UK strikes in Iraq - something which is not so clear cut in the case of attacks on IS strongholds in Syria, where the UK has made clear it will not co-operate with the regime of president Bashar Assad.

Any offer of military help to Iraq could lead to a recall of Parliament on Friday to seek MPs' approval, with Mr Cameron hopeful of avoiding the defeat he suffered last year over plans to target the Assad regime.

The bilateral meeting with Mr Rouhani reflects Mr Cameron's determination to involve regional powers in the struggle to defeat Islamic State, though he insisted that he would not temper his stance on Tehran's nuclear ambitions and support for groups which the West regards as terrorists.

"I will be very clear," Mr Cameron told NBC News. "We think they are wrong to have this nuclear weapon programme. We think they are wrong to support terrorist organisations.

"It'll be a tough conversation. I'm not saying that my enemy's enemy is my friend. I don't believe that.

"But the fact is if we want to have a successful, democratic, pluralistic Iraq and if we want to have a successful, democratic, pluralistic Syria, Iran can play a constructive role in helping to bring that about."

He said he hoped Mr Rouhani would make clear that Iran "can see the benefits of stability and the defeat of terrorism in that region".

Mr Cameron will also back a sweeping new UN resolution tabled by the US, which would impose global travel bans on foreign fighters planning to join conflicts overseas, as hundreds of Britons are believed to have done in Syria and Iraq.

The move, being personally pushed by Mr Obama at a Security Council meeting attended by Mr Cameron, would require UN member states to update national laws to stem the flow of foreign fighters, including by withholding passports and freezing assets.

The Pentagon said that a wave of bomb and missile attacks on targets in Syria and Iraq on Monday night was prompted in part by fears that a shadowy al Qaida-linked organisation known as the Khorasan Group was preparing to mount "imminent" terror attacks on Western targets.

The first US raids on targets in Syria involved Tomahawk cruise missiles fired from US warships at IS compounds, workshops and training grounds around the city of Aleppo, ahead of warplane and drone strikes on positions elsewhere in the country.

Among the areas reportedly hit was the IS stronghold of Raqqa where it is thought British aid worker Alan Henning is being held.

Mr Henning's brother-in-law Colin Livesey said he was "scared" about the possible impact of the air strikes on the former taxi driver's safety.

"If they're going to do air strikes on them, they'll just run away," Mr Livesey told ITV News. "They'll take him with them and no-one will know where he is again.

"He thought he was doing the right thing by going up there and helping other people in another country, to give them aid, because that's the sort of guy he is.

"He just went there pure and simple to help kids out. That's all he did it for. He doesn't deserve what's happening to him now."

Mr Henning's wife Barbara urged Islamic State to release him after she received an audio message from her husband pleading for his life.

Barbara Henning called for the militant group to "open their hears and minds" and said she had been told that a Sharia court had found her husband innocent of being a spy.

"I implore Islamic State to abide by the decisions of their own justice system.

"Please release Alan," she said in a statement released through the Foreign Office.

Following the raids, Mr Obama said the backing of five Arab states - Jordan, Qatar, Saudi Arabia, Bahrain and the United Arab Emirates - underlined the support he was building.

"The strength of this coalition makes it clear to the world that this is not America's fight alone," the president said in a statement on the White House lawn.

"The overall effort will take time, there will be challenges ahead, but we are going to do what's necessary to take the fight to this terrorist group for the security of the country and the region and for the entire world."

Since August, the US has carried out around 190 air strikes against IS in Iraq as part of Mr Obama's campaign to "degrade and destroy" the militants.

Britain has so far been helping supply arms to the Kurdish peshmergas fighting IS in northern Iraq, as well as offering support to the new Iraqi government formed by Mr Abadi earlier this month.

Labour leader Ed Miliband said his party was "open to the possibility" of UK forces joining air strikes in Iraq but would not commit in advance of seeing detailed proposals.

He said he would "look very seriously" at any proposal to meet a request from Iraq for British involvement, but action in Syria was a "more complex" question.

"Isil is a threat that can't be turned away from," he told BBC Breakfast.

"We've said that we need an alliance which is not just a military alliance but a political and diplomatic alliance.

"How will I judge any proposal? Whether Britain can have an effect, whether we can succeed and whether it is legitimate and lawful. But I am open to the possibility."

He went on: "This is a very grave decision as someone who wants to be the prime minister.

"Before I commit British combat troops I want to look at what the proposition is and the nature of that proposition."

He said he had not spoken to Mr Cameron about the issue or the potential recall of Parliament.

Asked about action in Syria, he said he understood why the US had intervened there but renewed his call for a United Nations Security Council resolution to give the action "proper legitimacy" in the absence of a request from the Assad regime.

Asked whether military strikes were needed, Home Secretary Theresa May, speaking in London, said: "From the Government's point of view in dealing with Isil, we've said very clearly we will ... look across the board at what steps we can take to deal with Isil, which does of course pose a threat to us here in the UK."

Asked if strikes would require legal justification, she added: "The whole situation regarding what action Government may or may not take, obviously it's subject for discussion. As you know the Prime Minister is over at New York at the UN General Assembly and I can assure that any action Government does take will have an appropriate legal basis."