Washington: A US citizen working in Syria with a militant group backed by Al Qaida conducted a suicide bombing there Sunday, in what is believed to be the first time an American has been involved in such an attack, US officials said Wednesday.

The suicide attack first surfaced Tuesday in Twitter messages from the Nusra Front, an Islamist extremist group in Syria aligned with Al Qaida in the fight against the government of President Bashar Al Assad of Syria.

US officials, speaking on condition of anonymity because intelligence matters were involved, declined to identify the American or provide any information about him. NBC News first reported that US officials had confirmed the bomber was an American.

Syrian activists and jihadist social media sites reported that the American went by the name Abu Huraira Al Amriki and carried out the suicide truck bombing in the northern province of Idlib.

A photo circulated on jihadist social media accounts showed a smiling young man who was said to be the bomber. He had a blond beard and was holding a cat to his chest.

Activists also circulated a video that was said to record the attack. It first showed rebels loading what appear to be tank shells into a large vehicle that had been armoured with metal plates. In the video, there is a large explosion after the vehicle drives down a road.

An anti-government activist reached through Skype near the bombing site confirmed the attack and said he had seen the American before the bombing but had not spoken to him and did not know where he was from.

“I know he was an American, had an American passport and that he was with the Nusra Front,” said the activist, who gave only his first name, Ahmad, for fear of retribution.

Islamist extremist groups in Syria with ties to Al Qaida have been trying to identify, recruit and train Americans and other Westerners who have travelled there to get them to carry out attacks when they return home, according to senior US intelligence and counterterrorism officials.

These efforts, which the officials say are in the early stages, are the latest challenge that the conflict in Syria has created, not just for Europe but for the United States. The civil war has become a magnet for Westerners seeking to fight with the rebels against the Al Assad government.

US intelligence and counterterrorism officials say more than 70 Americans have travelled to Syria, mainly to fight for one of the hundreds of rebels groups combating the Al Assad government. The FBI, CIA, National Counterterrorism Centre and Homeland Security Department recently created a special team of analysts to try to prevent the American jihadists from returning home undetected.

“The recent flood of militants into the country poses a serious challenge, as these individuals could be trained to plan and carry out attacks around the world,” Mark F. Giuliano, the deputy FBI director, said in prepared remarks in Washington on Wednesday.

A US suicide bomber in Syria is a “potential game changer,” said Seth Jones, a terrorism expert at the RAND Corp.

“It indicates that Americans are honing their ability to conduct terrorist attacks,” Jones said, and could eventually be used inside the United States or against US interests abroad.

Most of the Americans who have travelled to Syria are still there, US officials said, although a few have died on the battlefield. Nicole Lynn Mansfield, 33, of Flint, Michigan, a convert to Islam, was killed in May 2013 while with Syrian rebels in Idlib province.

Last year, another American, Eric G. Harroun, a former Army soldier from Phoenix, was indicted in Virginia by a federal grand jury on charges related to allegations that he fought alongside members of the Nusra Front. In September, he pleaded guilty to a lesser charge involving conspiracy to transfer defence articles and services, and was released from custody. Harroun’s family posted a notice on his Facebook page last month saying that he had died, apparently from an accidental overdose.