Pakistani police say they have arrested three Islamic extremists who planned to assassinate two US diplomats in a suicide attack by ramming their car with an explosives-laden Volkswagen bus. The arrests of the men, who were also said to be involved in a May bombing that killed 11 French engineers, then led police to the largest cache of explosives ever discovered in the terrorism-ravaged country, they said.

The three men were arrested Saturday with 10 kilograms (22 pounds) of explosives packed into a Volkswagen bug that was to be used in the attack in this southern port city, Sindh province's police chief Kamal Shah told reporters.

After interrogating the men police raided a warehouse in the east of the city where they found about 10,000 kilograms (22,000 pounds) of aluminium nitrate -- the largest ever cache of explosive material found in Pakistan and potentially enough to make thousands of bombs.

The aluminium nitrate was kept in 250 sacks in the warehouse, which also included detonators. Police were looking for the warehouse's owner.

The three arrested men were members of the extremist movement Harkat-e-Jihad-e-Islami and had received training in Afghanistan, he said.

The revelation came on the eve of a visit to Pakistan by US Assistant Secretary of State for South Asia Christina Rocca, who is expected to arrive in the capital Islamabad early Monday.

The plot against the diplomats will not affect Rocca's visit, said Terry White, spokesman for the US embassy in Islamabad.

A spokesman for the US consulate in Karachi said no further security measures were being taken at the mission, itself the target of a June 16 bombing that killed 12 Pakistanis. The consulate is already on extreme alert and operates from an undisclosed location.

Shah identified the would-be suicide bomber as Asif Zaheer, 28, and said he had backed out at the last minute from the May 8 suicide attack outside Karachi's Sheraton Hotel that killed 11 French engineers and three Pakistanis.

Shah said police were still searching for six people in the anti-French attack, including one who is believed to be the mastermind of the anti-Western violence that has plagued this city for the past year.

"There is one person who we believe is the mastermind in all major terrorist attacks in Karachi, from Daniel Pearl to the Sheraton blast including this incident, and he is the one who provides suicide bombers," he said.

"We are on it and pretty soon we will be able to arrest him."

Pearl, an American journalist for The Wall Street Journal, was abducted in January as he researched a story and was later murdered.

PHOTO CAPTION

Police officers display the recovered explosives and instruments for media at a news conference, Sunday, Dec. 15, 2002 in Karachi, Pakistan. Police arrested three men Friday and Saturday and seized about 250 sacks of ammonium nitrate, the fertilizer used in the Oklahoma City bombing. One of the suspects was also linked to a May suicide bombing outside a hotel in Karachi that killed 14 people, including 11 French engineers. (AP Photo/Shakeel Adil)
- Dec 15 1:02 PM ET