Al-Qaida in the Arabian Peninsula is sustained by factors including wild terrain and the weakness of the Yemeni state

Exactly what danger is posed to US, British and French interests in Yemen is not public knowledge. But it comes as no surprise that the poorest country in the Arab world and the home of al-Qaida's most active local "franchise" is the apparent focus of the international terrorist alert that has led to the closure of western embassies across the Middle East.

Al-Qaida in the Arabian Peninsula (AQAP) emerged in Yemen in 2007 after the organisation's effective defeat in neighbouring Saudi Arabia. It is the regular target of US drone strikes. Little is known about the links between it and al-Qaida central in Pakistan. But it is sustained by local factors including wild terrain, economic misery, tribal divisions and the weakness of the Yemeni state, battered by the Arab spring and the threat from secessionist movements.

AQAP is led by Nasser al-Wahayshi, a charismatic Yemeni jihadist who has created "a unified and cohesive militant organisation that has been involved not only in several transnational terrorist attacks but also in fighting an insurgency that has succeeded in capturing and controlling large areas of territory", according to Stratfor, an international security consultancy.

In recent weeks Wahayshi, 36, has reportedly been appointed to a senior al-Qaida position by Ayman al-Zawahiri, Osama bin Laden's Egyptian successor. Wahayshi, who was Bin Laden's private secretary in Afghanistan, fled to Iran in 2001 and was extradited to Yemen in 2003. In 2006 he escaped from a prison in Sana'a in a mass breakout that did much to invigorate the country's violent extremists.

The group has been under heavy pressure over the past 18 months. Its fighters have been pushed back to desert hideouts from much of the territory they captured in southern Yemen. Despite these setbacks, they have continued publishing an English‑language online magazine called Inspire, a magnet for jihadists from Pakistan to Mali.

AQAP is monitored by Saudi intelligence as well as the CIA and MI6, which both have liaison officers in Sana'a and Riyadh. The Nigerian underwear bomber, Umar Farouk Abdulmutallab, who tried to blow up a flight from Amsterdam to Detroit in 2009, was radicalised in Yemen while claiming to be there studying Arabic. Earlier that year the group tried to assassinate the Saudi security chief, Prince Muhammad bin Nayef, with a bomb concealed on the attacker's body.

AQAP regularly attacks Yemeni security and intelligence officers, with more than 60 killed in the past two years, according to the country's interior ministry.

It was revealed in February that the CIA was secretly using an airbase in Saudi Arabia to conduct its drone assassination campaign in Yemen. Diplomatic cables released by WikiLeaks had earlier exposed the scale of US covert involvement.

Last week the Yemeni president, Abd Rabbuh Mansour Hadi, met Barack Obama in Washington. The two leaders "reaffirmed their commitment to a strong counter-terrorism partnership, discussing a range of efforts to counter the threat to both countries posed by al-Qaida in the Arabian Peninsula", the White House said.

Al-Qaida's local 'franchises'

Somalia Harakat al-Shabaab. Offshoot of the Islamic Courts Union, which splintered after defeat in 2006 by the Somali transitional federal government. Describes itself as waging jihad against "enemies of Islam". Has kidnapped and killed foreign aid workers. Designated as a terrorist organisation by western governments. Of special concern to UK security services along with Nigeria's Boko Haram movement.

North Africa Al-Qaida in the Islamic Maghreb (Aqim). Formed when the Algerian Salafist Group for Preaching and Combat merged with al-Qaida in 2007. Active in Algeria, Mali, Niger, Mauritania and Libya. Involved in kidnappings and bombings. The In Amenas gas plant in the Algerian Sahara was seized by an AQIM faction in January.

Iraq Islamic State of Iraq. Al-Qaida branch established in 2006. Responsible for the deaths of thousands of Iraqis. By last year had reportedly doubled strength to about 2,500. Involved in recent spate of sectarian bombings. Hundreds of members freed in mass breakout from Abu Ghraib prison near Baghdad last month.

Syria Jabhat al-Nusra. Founded in 2012. Announced merger with al-Qaida in Iraq. But leader Abu Mohammad al-Golani affirmed allegiance to Ayman al-Zawahiri. Devoted to fighting regime of President Bashar al-Assad. Not known to have been involved in attacks outside Syria. Western governments fear growth of jihadist extremism because of the crisis.