US and British diplomatic personnel evacuated from Sana'a as Washington warns of 'specific and immediate threat'

Yemeni security forces remained on high alert on Tuesday night amid fears of an imminent attack by al-Qaida in the capital, Sana'a, after the US and Britain withdrew all embassy staff and again urged their citizens to leave the country. The US state department later described a "specific and immediate threat".

BBC Arabic quoted a Yemeni military official as saying that "extraordinary and unprecedented" security measures had been put in place, with armoured vehicles deployed around the presidential palace and other sensitive government and foreign installations in the capital.

Dozens of al-Qaida operatives are said to have streamed into Sana'a in the last few days, apparently to take part in a terrorist attack, the BBC said. The Yemeni claim could not be confirmed but it appeared consistent with US statements.

"Sana'a has been literally inundated with armed personnel and armoured vehicles to ensure that the military maintains a tight grip over all state and foreign interests," the Yemen Post reported.

Residents awoke to the sound of an aircraft, which appeared to be a US P-3 Orion, a manned surveillance plane, shortly before the embassy evacuations.

Hours earlier, Yemeni tribal sources and unnamed officials had reported two US drone strikes that killed four al-Qaida operatives in Marib, north-east of Sana'a, including a senior commander who was named by al-Jazeera as Salah al-Jumati. Last month, the second in command of al-Qaida in the Arabian Peninsula (AQAP), Said al-Shehri, was killed in a US drone strike.

US attacks have increased in recent months, attracting criticism that they are extrajudicial killings and also hit innocent people, fuelling a cycle of revenge. "With that kind of escalation, some reaction had to be expected," said Abdul-Ghani al-Iryani, a Yemeni development expert.

Yemeni tribal sources told Reuters that a military helicopter had been shot down as it fired rockets at gunmen suspected of involvement in blowing up oil pipelines. Eight people on board were killed.

The New York Times reported that US intelligence services had intercepted communications between Ayman al-Zawahiri, al-Qaida's leader, and the Yemeni head of AQAP, Nasser al-Wuhayshi.

The paper quoted counter-terrorism officials as saying that Zawahiri had recently promoted Wuhayshi to be the new "general manager" of the terrorist network, making him the second most important man in the organisation.

In response to the intercept the US closed a number of embassies and consulates over the weekend, temporarily shutting down 19 diplomatic outposts in the Middle East and Africa.

In London the Foreign Office said that all UK embassy staff had been temporarily withdrawn. The FCO also repeated its advice against all travel to Yemen, which was first issued in 2011.

The US air force transported state department personnel out of Sana'a early on Tuesday but called the operation "a reduction in staff", not an evacuation.

"There are no plans at this point to undergo an evacuation of US citizens," said US state department spokeswoman Jen Psaki. She would not say how many staff had left Yemen, whether the state department was considering additional "ordered departures" of personnel in other countries, nor how long the diplomats would remain at their temporary base in Germany before returning to Yemen.

A US military official confirmed that the US has three ships in the Gulf of Aden: the amphibious assault ship USS Kearsarge, the amphibious landing dock USS San Antonio, and the dock landing ship USS Carter Hall. The ships comprise a Navy-Marine Corps amphibious-ready group of approximately 5,000 troops and provide the US with something of an insurance policy should an attack occur. The official emphasised that the group was in the area on a routine deployment, having left the Red Sea several days ago, and was not deployed in response to the current terror threat.

The Yemeni government had already announced a plan to tighten security measures at foreign embassies – particularly those of western countries – sea ports and airports and other facilities of strategic importance, including oil pipelines and power grids. Sana'a has said that it is hunting 25 named AQAP operatives it suspects of planning attacks.

AQAP has been blamed for the foiled Christmas Day 2009 effort to bomb an airliner over Detroit and for explosives-laden parcels that were intercepted the following year aboard cargo flights.

There were signs that the Yemeni government was not pleased with the embassy closures. "Yemen has taken all necessary precautions to ensure the safety and security of foreign missions in … Sana'a," said a statement issued by its embassy in Washington.

"While the government of Yemen appreciates foreign governments' concern for the safety of their citizens, the evacuation of embassy staff serves the interests of extremists and undermines exceptional co-operation between Yemen and the international alliance against terrorism. Yemen remains strongly committed to the global effort to counter the threats of al-Qaida and its affiliates."

Psaki said that the US secretary of state, John Kerry, had spoken with Yemen's president, Abed Rabbo Mansour Hadi, on Monday night to thank him for his co-operation. She would not be drawn on Yemen's reaction to the withdrawal of embassy staff, saying only that the US had judged the operation to be necessary to protect its staff and citizens.

Gregory Johnsen, a US academic expert on Yemen, said that the Obama administration's strategy in Yemen was not working because it was using tactics from the different circumstances of Afghanistan and Iraq.

"The men that the United States is killing in Yemen are tied to the local society in a way that many of the fighters in Afghanistan never were," he wrote in Foreign Policy. "They may be al-Qaida members, but they are also fathers and sons, brothers and cousins, tribesmen and clansmen with friends and relatives.

"The United States can target and kill someone as a terrorist, only to have Yemenis take up arms to defend him as a tribesman. In time, many of these men are drawn to al-Qaida not out of any shared sense of ideology, but rather out of a desire to get revenge on the country that killed their fellow tribesman."

Psaki rebuffed such criticisms, saying that al-Qaida's core leadership in Pakistan had been "weakened, decimated". But she added: "We have consistently expressed a concern about [al-Qaida] affiliates … and the fact that they continue to present a threat to the US and its interests. We consider this one of the foremost national security threats we face."