US authorities last night said they had arrested one of al Qaeda's highest ranking operatives as he was on his way home to Iraq to plan future attacks.

According to a Pentagon spokesman Abdul al Hadi al Iraqi is being detained in Guantanamo Bay.

Al Iraqi was transferred to US Defence Department custody this week from the Central Intelligence Agency, Bryan Whitman, a Pentagon spokesman, said. He declined to say where or when al Iraqi was captured or by whom.

However, a US intelligence official said al Iraqi had been caught late last year in an operation involving many people in more than one country. News of al Iraqi's capture came on the same day the Saudi authorities claimed to have smashed an al Qaeda-linked plot with the arrest of 172 militants in the Gulf kingdom.

Paul Gimigliano, CIA spokesman, also declined to say when and where al Iraqi had been captured.

He called al Iraqi "a veteran jihadist" and said his capture was "a significant victory in the fight against terror - getting him off the street is good news".

The Pentagon described al Iraqi as an associate of al Qaeda leader Osama bin Laden and someone who may have been targeting Westerners outside of Iraq.

At one time he served in the Iraqi military, according to the Pentagon.

The Pentagon took custody of al Iraqi at Guantanamo Bay, the detention centre for terror suspects, Mr Whitman said. He is the fifteenth so-called high-value detainee to be taken to Guantanamo Bay on Cuba after being held by the CIA in secret prisons abroad.

The other 14 were sent to Guantanamo Bay last September and have since undergone military hearings to affirm their status as enemy combatants eligible for military trials.

Mr Whitman said al Iraqi was believed responsible for plotting cross-border attacks from Pakistan on US forces in Afghanistan, and that he led an effort to assassinate Pakistan's President, General Pervez Musharraf, as well as unspecified officials of the UN.

He added: "Abdul al Hadi (al Iraqi) was trying to return to his native country, Iraq, to manage al Qaeda's affairs and possibly focus on operations outside Iraq against Western targets."

In Pakistan, Aftab Khan Sherpao, Interior Minister, described the arrest of al Iraqi as a "welcome development", but gave no indication that Pakistan played a role in it.

A senior Pakistani intelligence official said al Iraqi had operated from Pakistan's tribal regions but disappeared during the summer of 2005.

The official said al Iraqi had commanded al Qaeda forces when they fought US forces in Afghanistan's Shah-e-Kot valley in March 2002.

The Pentagon said al Iraqi was born in Mosul, in northern Iraq, in 1961. It said he was a key al Qaeda paramilitary leader in Afghanistan in the late 1990s, and during 2002-04 led efforts to attack US forces in Afghanistan with terrorists based in Pakistan.

In August 2005, al Iraqi appeared in a purported al Qaeda-made video that showed militants in Afghanistan - including Europeans, Arabs and others - preparing to attack US troops and showing off what they said was a US military laptop. Meanwhile, an al Qaeda-linked plot to attack oil installations and military bases was foiled when Saudi Arabian authorities arrested 172 Islamic militants, officials from the Gulf kingdom's Interior Ministry claimed yesterday.

The ministry said: "One of their main targets was to carry out suicide attacks against public figures and oil installations and to target military bases inside and outside (the country)."