The Islamic State group released a video purporting to show a young boy executing an Arab Israeli who infiltrated it in Syria to spy for the Jewish state.

In the video, a youth identifying himself as 19-year-old Mohammed Said Ismail Musallam recounts how he was recruited by Israeli intelligence.

Dressed in an orange jumpsuit that is standard in videos of IS executions, he is shown kneeling in front of the boy, who appears to be no more than 12 years old, and a man standing at his side.

The man, speaking in French, issues threats against Jews in France, before it appears that the boy walks around in front of the hostage and shoots him in the forehead with a pistol.

The boy, who shouts 'Allahu Akbar' ('God is greatest'), then shoots the man four more times as he lies on the ground.

However, the video has been edited in such a way that the boy is not actually seen firing the gun.

In February, Musallam's father denied in comments to AFP the IS claim that his son was an Israeli agent.

"My son is innocent; IS accused him of working for Mossad because he tried to run away," Said Musallam said, claiming that his son had travelled to Syria to join the jihadists.

"He is absolutely not religious," Musallam said, adding that perhaps his son had been recruited through the Internet.

Dabiq, the IS online English-language magazine, said Mohammed had been tasked by Israel with gathering information on IS bases and weapons and on Palestinians volunteering to fight for the group in Syria.

A spokesman for Israel's Shin Bet security service told AFP Mohammed had left home on October 24 "on his own initiative", implying that he had not been sent by Israel.

Official Israeli policy is not to comment on such allegations.

The Shin Bet spokesman said Mohammed travelled from Israel to Turkey and "from there crossed to Syria and joined IS to fight for the organisation".

Said Musallam said his son, an Israeli citizen, abandoned his national service in the fire department to join IS.

He said the last time they spoke by phone Mohammed was in Raqa, the self-proclaimed capital of the jihadist "caliphate" in northern Syria.

He said his son wanted to return home after having completing basic training.

He said he was very worried about his son's fate at the hands of IS, which has brutally executed many prisoners in recent months.

Asked if he could raise a ransom in exchange for his son, the father said he was a mere bus driver with no property or assets to his name.

The family lives in the Jewish settlement neighbourhood of Neve Yaakov in Israeli-annexed east Jerusalem.

Said Musallam was born in the occupied West Bank but said his four children have Israeli passports.

According to Dabiq, Mohammed was recruited in Neve Yaakov by a Jewish neighbour working for the security services and was given away by his behaviour and his refusal to obey a commander.

Islamic State blows up key Tikrit bridge

The Islamic State group blew up the only bridge over the Tigris river in the entire Tikrit area as Iraqi forces continued to seal off the city.

"The bridge was blown up by Daesh," a police colonel said, using an Arab acronym for the jihadist organisation.

"A whole segment at the western end of the bridge collapsed."

An army lieutenant colonel said: "Their goal is to slow the advance of Iraqi troops because the bridge is the only way into Tikrit from the east."

The village of Albu Ajil, which Iraqi forces retook on Sunday, is on the eastern side of the river, as is the town of Al-Alam, where jihadist fighters expelled from rural areas have been regrouping.

Ad-Dawr, the other town where IS fighters have been trying to resist the huge operation launched on March 2 to retake Tikrit, lies south of the city on the eastern bank of the Tigris.

Commanders from the army and the government-controlled Popular Mobilisation units have been closing in on the three urban centres over the past week.

They have said their goal was to lay siege to Tikrit, a Sunni city about 160km north of Baghdad which has been under IS control for nine months.

The city, which is the home town of former president Saddam Hussein, is the toughest target for the government troops and allied militias that started winning back lost ground last year.

The operation initially involved 30,000 men backed by Iraqi aircraft.

The IS is believed to have only a few hundred men inside Tikrit, but government forces have said their advance has been slowed by large numbers of roadside bombs and booby traps planted by the jihadists all around the city.