The claim by Umar Farouk Abdulmutallab that he was trained, armed and tasked with blowing up an American airliner over US soil by al-Qaida operatives based in Yemen is the western intelligence community's worst nightmare come true.

Since the September 11 attacks, the US and allied security services hunting Osama bin Laden and his associates have focused their attention on the mountains of eastern Afghanistan and the lawless tribal areas of Pakistan.

In recent years, concern has grown about the activities of al-Qaida affiliates in North Africa's Maghreb region, notably Algeria, and in a few sub-Saharan countries such as Nigeria, birthplace of the would-be bomber of Northwest Airlines flight 253.

But in all these cases – even Afghanistan, once its Taliban rulers were deposed – the US could count on the collaboration of established, friendly governments that felt equally threatened by the spectre of Islamist terrorism.

Yemen, and Somalia, its terrible twin situated just across the Gulf of Aden, are a different matter altogether.

Both countries lack effective central government. Both, having suffered a long history of colonial intervention, are currently prey to warring factions that have no love of the west.

And both contain vast, so-called "ungoverned spaces" that offer ideal hideouts and training centres for "non-state actors", the intelligence community's polite euphemism for terrorists.

Large tracts of sparsely populated Yemen are, in effect, "no-go" areas for the forces of global counter-terrorism. These safe havens remain mostly out of sight and, despite a trailblazing CIA Predator drone attack against al-Qaida in 2002, mostly out of range.

In short, Yemen has become the international jihadi's destination of choice from which to prepare, plot and launch future terror attacks. "Only Pakistan's tribal regions rival Yemen as a terrorist Shangri-La", the Wall Street Journal said this year, citing American estimates that up to 1,500 al-Qaida-linked fighters are based there.

Now Abdulmutallab, the well-to-do, well-educated Nigerian recruit, has demonstrated what the Yemeni terrorist melting pot is capable of producing – and just how far its malice can reach.

The signs have been there for those who wished to read them. In an under-reported incident in August, a suicide bomber crossed from Yemen into pro-western Saudi Arabia, passed two security checks, and blew himself up only yards from Prince Mohammad bin Nayef, the Saudi counter-terrorism chief.

The same military explosive, pentaerythritol, that Abdulmutallab attached to his leg was used by the bomber in the Saudi attack, though the latter concealed it in his rectum. Like the Northwest passengers, Nayef escaped serious injury.

Another grim message of intent came in October when al-Qaida's Yemen-based "emir of the Arabian peninsula", Nasir al-Wahayshi, urged supporters to use any means to kill western unbelievers. He identified preferred targets. They were "airports in the western crusade countries that participated in the war against Muslims; or on their planes".

Abdulmutallab's statement to the FBI that he went to Yemen this year and received instructions from al-Qaida there is now under investigation by the government in Sana'a, which said it was co-operating fully with the US.

"The whereabouts and exact details of what he did in Yemen are still unknown, but the investigation will clear up these things in the coming days," a Yemeni official said.

Despite their Af-Pak focus, the US and allies such as Britain have not ignored the Yemen threat. In September, John Brennan, the White House counter-terrorism chief, travelled to Sana'a, and in an unusually strong statement, Barack Obama declared the security of Yemen to be "vital for the security of the United States".

Since then, Washington has provided unspecified assistance to Yemeni and Saudi crackdowns on jihadi bases and Iranian-backed Shia rebels, amid unconfirmed reports that US special forces are in the country.

Two air strikes on al-Qaida strongholds in Yemen, the latest on Christmas Eve, reportedly killed up to 60 militants. It remains unclear whether these unusual operations were influenced by knowledge of a plot to blow up a US airliner.

Yet such efforts notwithstanding, the fact remains that Yemen's security problems retain potential to destabilise Saudi Arabia and other Gulf states.

Author Christopher Boucek, in a report this year by the Carnegie Endowment for International Peace, warned: "The inability of the Yemeni central government to fully control its territory will create space for violent extremists to regroup and launch attacks against domestic and international targets."

Another uncomfortable fact is that Abdulmutallab pointed to western military intervention in Afghanistan as the justification for his actions. His words appear further to undercut always tendentious official arguments that the war is making western countries safer.

Next time Gordon Brown tries to explain his Afghan policy, he may do well to examine its connection to what so nearly happened to Northwest Airlines flight 253 in the skies over Detroit.