Sanaa checks
A policeman inspects a van for explosives as Sanaa goes into lockdown [Reuters].

US drones are reportedly gearing up to bombard al-Qaida militants in Sanaa after dozens of insurgents reportedly infiltrated the Yemeni capital.

Scores of unmanned aircraft are said to have been hovering above the skies of Sanaa, whose Western diplomatic missions have already been evacuated.

Local reports suggest several drones have been spotted throughout the day as residents fear strikes could be unleashed any moment. Yemeni intelligence authorities have admitted dozens of al-Qaida militants have entered Sanaa aiming to stage a large-scale attack on Western diplomatic posts and government buildings.

Yemeni officials have confirmed that the earlier drone strike in the restive Marib province, a hotbed for al-Qaida operatives, killed four insurgents. The authorities flagged them as dangerous terror elements without releasing their identities.

Sources have also told the BBC's Newsnight that Washington is mulling whether to launch intense strikes against al-Qaida forces in Yemen.

Yemeni forces have beefed up their own defence measures in the capital, treating any security breach as a potential terror attack.

Scores of troops and tanks have been pouring into the capital to guard diplomatic missions and state institutions. Hundreds of armoured vehicles have been deployed in Sanaa, and the Yemeni administration has imposed a military lockdown.

However the Yemeni government has criticised the Western nations' decision to evacuate embassy staff.

"While the government of Yemen appreciates foreign governments' concern for the safety of their citizens, the evacuation of embassy staff serves the interests of the extremists," said Yemen's foreign ministry in a statement.

The latest threats have emanated from al-Qaida's Yemeni arm, Al-Qaida in the Arabian Peninsula (AQAP). American officials said they intercepted conversations between al-Qaida chief Ayman al-Zawahiri and the AQAP head, but refused to divulge specifics.

The Yemeni arm of al-Qaida is widely believed to be the affiliate most capable of launching high-profile attacks.