Intelligence reports indicate al-Qaeda's top bombmaker in Yemen, Ibrahim Hassan Al-Asiri, could be behind the threats that have prompted the United States to tighten airline security.

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Prime Minister David Cameron said the safety of passengers "must come first" as the UK complied with requests from the United States to step up airport security to prevent attacks from militants in Yemen and Syria.

Intelligence reports suggest that al-Qaeda-linked groups in Yemen and Syria may have joined forced to plot an attack on flights travelling directly to the US.

Mr Cameron said: "The safety of the travelling public must come first. We mustn't take any risks with that. I hope this won't lead to unnecessary delays but it's very important that we always put safety first, and we do."

Channel 4 News Foreign Affairs Correspondent Jonathan Rugman writes in his blog that Yemen's top bombaker, Ibrahim Hassan Al-Asiri, could be behind the threats, using jihadists in Syria with European passports to become bomb carriers.

"There are too many jihadists going and coming back to keep tabs on. If any of them are students of Mr Al-Asiri, the terrorist threat could increase exponentially," he suggests.

"Mr Al-Asiri rarely makes the headlines these days, but for several years now he has laid claim to being the most world’s most dangerous man, if you live in Britain or the United States."

Al-Asiri has a past history of attempting to make bombs that could not be discovered, including one that was used in a suicide bombing by his brother Abdullah in an attempt to kill Saudi Arabia's head of intelligence. It is believed Abdullah Al-Asiri had hidden the bomb inside his rectum when he detonated it.

"The UK terror threat level has not increased - which makes it clear there is no specific intelligence about a specific attack. Small comfort though, if as it appears Al-Asiri is still trying to perfect a bomb which is undetectable," suggests our correspondent.

Yemeni's top bombmaker: the world's most dangerous man? 
A nightmare for western security officials is when one "ungoverned space" links up with another, so that intercepting terror threats becomes a game of “whack a mole” – concentrate on Location A and Location B suddenly causes you problems instead.
This seems to be what is happening with al-Qaeda in the Arabian Peninsula or AQAP, which has reportedly tried to cooperate with Jabhat Al Nusra, another jihadist group in Syria.
Read Jonathan Rugman's blog about the man apparently behind the latest airport security scare
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The enhanced security measures are expected to come in within the next few days. The US has not specified which airports or countries will be affected, or what triggered the new alerts.

The new security measures could include more bomb-detection machines as well as additional inspections of passengers' shoes and property, and come just ahead of the summer holiday rush.

An official said Washington had the legal power to enforce security upgrades on foreign airports because the flights go directly to the US.