Image caption The prime minister was reporting back to MPs after the G20 summit in Australia

The UK will not be "cowed" after Islamic State killed another western hostage, David Cameron has told MPs.

The prime minister described IS as "sick terrorists" who he pledged would be defeated.

A video was released on Saturday showing the beheading of a US aid worker and 18 Syrian prisoners.

Earlier Home Secretary Theresa May told the Commons the killing of American Abdul-Rahman Kassig demonstrated the "deadly threat" faced by the UK.

Mr Kassig - known as Peter before he converted to Islam - became the fifth western captive beheaded by IS in recent months.

The father of a British man thought to be pictured in the video has denied it is his son.

Ahmed Muthana said 20-year-old Nasser Muthana was not among the uniformed jihadists seen in the footage.

'Deadly threat'

Meanwhile the Ministry of Defence said it had carried out further air strikes against the militants in Iraq.

It released a video which it said showed a single RAF missile strike on an IS communications vehicle.

Image copyright PA Image caption Home Secretary Theresa May: "This demonstrates the very deadly threat we face from terrorism"

Appearing in the Commons after discussing the West's response to IS at the G20 in Australia, Mr Cameron said: "We will not be cowed by these sick terrorists. They will be defeated and they must face the justice that they deserve.

"The threat is faced by countries right across the world. We must face it together."

Mr Cameron said new legislation aimed at tackling British citizens going abroad to fight with the militants would be introduced in the next two weeks.

The Counter-Terrorism Bill would give police new powers to seize passports, stop suspects from travelling, and to stop Britons returning to the UK "unless they do so on our terms", he said.

Mrs May said Mr Kassig's murder, and the recent attack on the Canadian parliament, were reminders of "the very deadly threat we face from terrorism at home and abroad".

She told MPs: "That is why protecting the British public remains this government's number one priority and why we're taking urgent action to ensure our police and intelligence agencies have all the tools they need to keep people safe."

Labour leader Ed Miliband said the latest beheading was another demonstration of IS's "evil ideology" that reinforced "our determination" to defeat the militants.