A chilling video depicting the apparent murder of a U.S. journalist by jihadists is just the latest salvo in an online war being waged by extremists on social media sites.

Jihadist groups have long used their own media organizations to distribute messages and videos, but in recent years platforms like Twitter have given them an unprecedented, unfiltered ability to intimidate their opponents and recruit members.

Syrian troops battled Wednesday to hold onto their last stronghold in the northern province of Raqa, under fierce attack from Islamic State jihadists, a monitoring group said.

The Syrian Observatory for Human Rights said IS fighters had been attacking Tabqa military airport for around 10 days, but the fighting overnight from Tuesday was the heaviest yet.

U.S. President Barack Obama called for a joint effort to eliminate the "cancer" of jihadist terror in Iraq and Syria on Wednesday, after Islamic State militants murdered an American journalist.

Obama said the entire world was appalled by the beheading of 40-year-old reporter James Foley, which the IS fighters videotaped and published on the Internet.

A delegation from Uruguay will head to Lebanon on Friday to select 120 Syrian refugees who can go to the South American country, the country's Human Rights Department said.

Forty of the refugees are scheduled to arrive in September and the rest next year.

Jihadists released a video apparently showing the beheading of an American journalist kidnapped in Syria, in the most direct retaliation yet to U.S. air strikes against them in Iraq.

The video, in which the Islamic State (IS) jihadist group also threatens to kill another reporter if U.S. air strikes in Iraq continue, sparked global outrage on Wednesday.

A Jordanian air force captain who joined jihadists in Syria to fight against President Bashar Assad's forces was killed in battle on Tuesday, a Salafist source said.

Ahmad Atallah Shbeib al-Majali reportedly deserted to fight alongside Al-Nusra Front, the Syrian franchise of Al-Qaeda, before joining the extremist Islamic State group.

The jihadist Islamic State has more that 50,000 fighters in Syria and recruited 6,000 last month alone, the Britain-based Syrian Observatory for Human Rights said on Tuesday.

The group, which relies on activists, doctors and lawyers on the ground in Syria for its reporting, said July saw the Islamic State's largest recruitment yet.

Saudi Grand Mufti Sheikh Abdul Aziz al-Sheikh on Tuesday blasted al-Qaida and Islamic State jihadists as "enemy number one" of Islam, in a statement issued in Riyadh.

"The ideas of extremism, radicalism and terrorism... have nothing to do with Islam and (their proponents) are the enemy number one of Islam," the kingdom's top cleric said.

The U.N. refugee agency said Tuesday it was poised to mount a massive aid operation for 500,000 Iraqis driven from their homes by jihadist rebels.

"In response to the deteriorating situation in northern Iraq, UNHCR is this week launching one of its largest aid pushes aimed at helping close to half a million people who have been forced to leave their homes," spokesman Adrian Edwards told reporters.

Iraqi forces battled Sunni militants along a string of fronts on Tuesday, including at Saddam Hussein's hometown Tikrit, as the United Nations readied a massive aid operation for displaced Iraqis.

Kurdish and federal forces, who wrested back control of Iraq's largest dam, fought jihadists in the country's north, buoyed by intensifying U.S. air strikes and Western arms deliveries.