The family of murdered US journalist Steven Sotloff has paid a moving tribute to their son, remembering a gentle soul who was fiercely committed to giving "a voice to those who had none."

The 31-year-old had been drawn to reporting from the world's conflict zones because he was unable to "turn his back on the suffering pervading the world," a family spokesman said.

Mr Sotloff's father briefly appeared holding a photo of his son, declining to speak to media.

His horrific killing by the Islamic State was shown in a video which emerged on Tuesday.

It was posted online, just weeks after fellow journalist James Foley was murdered by the group in near-identical circumstances.

His family spokesman Barak Barfi said in a statement the reporter had been attracted to Syria through a fascination with the Arab world.

"He was no war junkie, he did not want to be a modern-day Lawrence of Arabia: he merely wanted to give voice to those who had none," Mr Barfi said.

"From the Libyan doctor who struggled with psychological services to children ravaged by war to the Syrian plumber who risked his life by crossing regime lines to purchase medicine, their story was Steve's story.

"He ultimately sacrificed his life to bring their story to the world," added Mr Barfi, insisting he was "no hero."

"Like all of us, he was a mere man who tried to find good concealed in a world of darkness and if it did not exist he tried to create it."

Mr Barfi ended the statement with off-the-cuff remarks in Arabic, saying "Steve died a martyr for the sake of God."

He then challenged Islamic State leader Abu Bakr al-Baghdadi to debate Islam, saying: "Woe to you. You said the month of Ramadan is the month of mercy. Where is your mercy?"

"God does not love the aggressor," added Mr Barfi, who is an Arabic scholar and research fellow at the New America Foundation think tank in Washington.

He went on: "I am ready to debate you with kind preachings. I have no sword in my hand and I am ready for your answer."

Mr Sotloff, well-versed in the history and culture of the Middle East, was taken captive in Syria last year.

The self-styled "stand-up philosopher from Miami," who wrote for Time, the Christian Science Monitor, Foreign Policy and World Affairs Journal, had always found time for family despite his workload.

Mr Barfi said "This week, we mourn, but we will emerge from this ordeal. Our village is strong. We will not allow our enemies to hold us hostage with the sole weapons they possess: fear."

NATO would consider Iraq request for IS help

NATO would "seriously" consider any request from Iraq for assistance in a war against insurgents from the radical Islamic State, NATO chief Anders Fogh Rasmussen said today.

"We will discuss what individual allies and what NATO can do to counter the threat from the terrorist organisation, so-called Islamic State."

His comments came before the start of a two-day summit of leaders from the Western military alliance in Newport, Wales.

"We haven't received any request for NATO engagement. I'm sure that if the Iraqi government were to forward a request for NATO assistance, that would be considered seriously by NATO allies," the secretary general said.

"I do believe that the international community as a whole has an obligation to stop the Islamic State from advancing further," he said, after two US journalists were shown being beheaded in videos by extremists as retaliation for US air strikes in Iraq.

One of the videos also said a British hostage would be killed next if bombing continued.

British Prime Minister David Cameron and US President Barack Obama said in an op-ed in the Times newspaper that their countries would not be "cowed" by the threats.

IS capture dozens in northern Iraq

Jihadists kidnapped dozens of residents of a northern Iraq village today, after villagers burned one of their positions along with a jihadist flag, police and witnesses said.

The militants of the Islamic State (IS) group had withdrawn from Tal Ali in Kirkuk province yesterday, but returned in force today and abducted some 50 people, the sources said.

It is not the first time IS has carried out mass kidnappings in Iraq.

The group abducted thousands of civilians as it overran minority-populated northern villages last month, according to human rights group Amnesty International.

Amnesty has accused IS of  "systematic ethnic cleansing," including mass killings, of ethnic and religious minorities in Iraq.

A senior UN rights official has said the group is responsible for "acts of inhumanity on an unimaginable scale."

IS-led militants launched a lightning offensive in the north in June, sweeping through much of the Sunni Arab heartland north and west of Baghdad before turning on Christian and Yazidi areas.

Iraqi security forces, now bolstered by thousands of Shia militiamen as well as Kurdish fighters, have clawed back some ground northeast of Baghdad.

But significant areas, including parts of Kirkuk province, remain under militant control.