The US says an Islamic State chemical weapons expert was killed in a coalition air strike in northern Iraq last week.
A statement from US Central Command, which controls US military operations in the Middle East, said the death would "temporarily degrade and disrupt the terrorist network".
Abu Malik had been a chemical weapons engineer during the rule of Saddam Hussein and joined al-Qaeda Iraq in 2005, Central Command said.
Sir John Chilcot is to give evidence to the House of Commons Foreign Affairs Committee on 4th February about his delayed report into the Iraq war, the committee said.
Earlier, this month he said it was unlikely that his long-awaited report would be published before the general election.
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Hundreds of guerrilla attacks have been carried out on Islamic State militants by a network of "commando cells" working under the radar, NBC News reports.
The little-known resistance movement, dubbed the Mosul Liberation Battalions, uses sniper attacks, homemade bombs and weapons stolen from the militants' storage sites to target the terror group, which took hold of the city in June last year.
The NBC News report states that Iraq's vice president, Osama al-Nujaifi, revealed "a large number" of Islamic State fighters had been killed in more than 300 operations, carried out by volunteers including former military officers and tribesmen as well as students and local businessmen.
Tackling the threat posed by Islamic State militants is "the challenge of our time", US Secretary of State John Kerry has said.
Arriving in London for a summit of foreign ministries from around the world, Mr Kerry said there had been "sleeper cells" of extremists in place for a long time, planning attacks which were only now coming to fruition.
He said the aim of the militants was to attack Western values.
The truth is that these groups are planning, and have been planning for a long time - going back to Osama Bin Laden and 9/11 in New York - to attack Western interest and go after anybody that they disagree with.
A very neolistic, unbelievably oppressive sense of how people ought to live. We've seen them carried out in the most grievously horrendous fashion, with public beheadings.
I think this is a challenge for all of us - it's the challenge of our time.
The Prime Minister has promised the UK will do "everything [it] can" to stop people travelling to Iraq to join Islamic State fighters.
Speaking in No. 10, David Cameron told his Iraqi counterpart Haider al-Abadi:
The threat from extremist terror you face in Iraq is also a threat we face here in the United Kingdom.
We will do everything we can to help stop foreign fighters coming to your country and creating the mayhem we see today.
Islamic State militants could begin orchestrating terror attacks on the UK and other Western countries from their base in Syria unless urgent action is taken, the Foreign Secretary has warned.
Speaking to ITV's Good Morning Britain, Philip Hammond said the biggest threat to security currently was the potential for "lone wolf" attacks inspired by jihadist activities overseas.
But if extremists managed to keep hold of their territory in the Middle East, he warned, the UK could expect to see organised attacks being planned and executed from afar.
It comes as a meeting is set to be held at Lancaster House in London to discuss the global response to the terrorist group.
Islamic State militants seeking to "subvert and destroy the values of democracy" must be defeated, the Foreign Secretary has warned - saying action was crucial "for the security of Britain."
Foreign ministers from around the world were due to fly in to London today for a top-level summit discussing how to take the terror group down.
US secretary of state John Kerry, Iraqi prime minister Haider al-Abadi and French foreign minister Laurent Fabius will be among leaders from 21 countries joining the talks.
They will focus not only on military support for opposition fighters but ways to cut off Islamic State finances, stop its influence spreading and preventing foreign fighters joining their ranks.
In an article for the Daily Telegraph, Philip Hammond said:
Some say that Britain should leave this fight to others. But we cannot sub-contract the safeguarding of our nation's security.
Nor can we ignore an organisation that seeks to subvert and destroy the values of democracy, free speech, freedom of religion and the rule of law that are so fundamental to our way of life.