RIYADH (AFP) - Saudi Arabia has arrested two suspected members of Al-Qaeda who may have been plotting against Western embassies in the Middle East, the interior ministry said.

But while the focus of a region-wide alert remains on Al-Qaeda in the Arabian Peninsula, the United States late on Thursday pulled all non-emergency staff from its Lahore consulate in Pakistan.

The two men who were arrested, a Yemeni and a Chadian national, had contacts with AQAP, the terror network's Yemeni offshoot, state news agency SPA quoted ministry spokesman General Mansour al-Turki as saying on Thursday.

The Chadian suspect had been expelled from Saudi Arabia but returned with a passport issued by another country, Turki added.

"The two suspects may have been implicated in the threats against Western embassies in the region," he said.

Al-Qaeda in the Arabian Peninsula - which has been at the heart of the security alert that has shut a number of US and Western missions in the Middle East, Asia and Africa - is seen by Washington as the most active branch of the jihadist network.

It was formed in January 2009 as a merger of the Yemeni and Saudi branches of Al-Qaeda and is led by Nasser al-Wuhayshi.

According to a report in the Wall Street Journal citing an anonymous US official, Wuhayshi masterminded the latest plot.

Previous reports have said that he was ordered to go on the offensive by Al-Qaeda's overall leader, the late Osama Bin Laden's former number two Ayman al-Zawahiri.

But the official who briefed the Journal said that an intercepted communication had shown that Zawahiri merely approved an operation that had been drawn up in Yemen.

"Zawahiri's giving his blessing for a plot is very different from ordering that plot or being able to launch a 9/11-style attack," the official said, according to the report.

If confirmed, the idea that Zawahiri played a passive role in the plot will lend weight to US President Barack Obama's claim that so-called "core Al-Qaeda" is on the ropes.

But it will also feed fears that regional Al-Qaeda franchises, such as Wuhayshi's AQAP, are becoming major threats in their own right.

However, that narrative suffered a blow late Thursday when the US State Department withdrew staff from its Lahore consulate, citing "specific threats." A spokeswoman for the US embassy in Islamabad said the evacuation was not linked to the alert that prompted the closure of embassies and consulates in the Middle East and Africa.

Despite Pakistan's fractious alliance with the United States in the "war on terror", anti-American sentiment runs deep in the restive country, fuelled in part by the CIA's campaign of drone strikes against militants in the tribal northwest.

It had previously been reported that in the communications, intercepted by US intelligence, Zawahiri named Wuhayshi as operational controller of all Al-Qaeda groups.

According to Yemeni officials, his group planned to storm a Western-run oil terminal and seize a port city.

US officials have not said what they think the targets were, but they have closed 19 embassies and diplomatic missions until at least the end of the week.

Yemeni officials believe they have foiled the plot, and there have been several reported US drone strikes this week.

The latest trio of strikes on Thursday killed 12 Al-Qaeda militants, according to a Yemeni military official and tribal sources.