Shortly before Abdul Waheed Majeed, a 41-year-old British truck driver, blew himself up in an attack on a Syrian prison, he brushed aside a question in Arabic.

"I'm sorry, I can't speak it," he said in a video. "My tongue bro'... it's got like a knot in it."

That suicide-bomb attack on February 6 by the Pakistani-born Majeed, appeared to be part of a resurgence of such attacks that represented a disturbing shift in tactics among radical jihadists in the sectarian killing grounds of Syria and Iraq.

Many of them have been carried out by foreigners drawn to the conflicts from across the region and from Europe, US and European security and intelligence officials say.

Will McCants, an expert at the Brookings Institution think tank in Washington, said given the rapid increase of foreign fighters in Syria "if the war drags on, the number of fighters will far eclipse those we saw in Afghanistan."

The security officials estimated that several thousand foreign nationals are active in the two countries.

Most are with the Islamic State of Iraq and the Levant (ISIL), an ultra-conservative militant group reconstituted from an earlier incarnation of al Qaeda and is active in Syria and Iraq and with Jabhat al Nusrah, an al Qaeda affiliate which is one of the most powerful rebel forces in Syria.

In the last year the rate of suicide bomb attacks in Iraq has risen sharply, back to levels not seen since 2007, US officials said.

The officials said they did not have precise data on the number of foreign fighters involved in the violence. But in March and April alone, at least 14 Tunisians fighting with ISIL blew themselves up at various locations in Iraq, according to postings on social media sites affiliated with ISIL, which US and European authorities monitor.

That is about half of the total number of foreign suicide bombers identified with ISIL on social media who blew themselves up during the two-month period, said Laith Alkhouri, a senior analyst with Flashpoint Partners, a group which monitors militant social media postings.

Other suicide bombers in Iraq in March and April included fighters from al Qaeda in the Arabian Peninsula, one of the group's most lethal wings based mostly in Yemen.

Alkhouri said many of them appeared to be Saudis, Libyans, Egyptians, Moroccans and Jordanians. A Danish citizen and a Tadjik were also reported by the group to have blown themselves up.

During the 2006-2007 civil war in Iraq, when the use of suicide bombings were rampant, foreigners made up the largest proportion of the militants carrying them out. After the US "surge" in 2007, when it rapidly built up its forces, the number of attacks dropped as sectarian violence as a whole waned.

Militant attacks are now increasing sharply again in Iraq as the powerful ISIL seeks to impose strict sharia law in the Sunni majority populated regions of the country.

In Syria, the conflict took on a regional dimension and attracted foreign fighters soon after it began with an outbreak of a mostly Sunni popular uprising in 2011 against President Bashar al-Assad.

Iran and Saudi Arabia used Syria as the front line in their Shi'ite-Sunni war for supremacy in the Arab world, said experts.

Foreign Sunni fighters have converged on Syria to fight alongside Sunni rebel forces, while Shi'ites from Iraq and Lebanon have joined Assad's forces.

Foreign fighters are coming from several Gulf countries besides Yemen, Kuwait, Tunisia, Libya, Egypt, Morocco, Jordan, Bosnia and other Arab states. Also fighters from Chechnya, Pakistan, Afghanistan and Western countries are joining rebels to take on Assad's army, according to several US and European security officials.

With Assad using his full firepower against rebels who lack sophisticated arms, the military balance tipped against the rebels last year, driving foreign fighters to carry out suicide attacks to make up for losses on the battleground.

English-speakers play a prominent role among foreign fighters. British security sources estimate that at least 400 British nationals have moved in and out of the Syrian conflict, with as many as 250 Britons on the battlefield at any one time.

The video of the British suicide bomber shows what appears to be a truck elaborately rigged as a suicide bomb. Standing nearby is a person who investigators identified as Majeed posing and talking with what appear to be fellow militant fighters.

While the video does not show him driving the truck into the prison, investigators say they believe he was the driver and that the video is authentic.

Perhaps fewer than a hundred Canadians and Americans have also joined Syria-based militant groups, say US intelligence officials. They estimate the number of Americans who have joined the conflict in the "dozens".

In an April 23 report, the Dutch AIVD intelligence service said it was aware of two Dutch nationals who became suicide bombers in the last year while fighting with militants. One blew himself up in Syria and the other in Iraq, said a European security official briefed on the matter.

Another European official said that at least one or two German nationals were believed to have been involved in recent suicide attacks.-Reuters