Police said today that they had foiled a suicide bomb plot by Islamic militants to ram an explosives-laden Volkswagen into a car carrying U.S. diplomats in Karachi.

Police said they arrested three men Friday and Saturday and seized about 250 sacks of ammonium nitrate, a fertilizer used in explosives. They said the suspects had been trained at a camp in Afghanistan run by Islamic militants fighting Indian rule in the disputed region of Kashmir.

"They were preparing for the attack on American diplomats in the coming days, and they had the car bomb ready," the provincial police chief, Kamal Shah, said at a news conference.

The suspects were surveying one of the main roads in this southern port city, frequently used by diplomats, for an appropriate site to attack, he said.

Shah said one of the suspects, Asif Zaheer, was linked to a suicide bombing outside a Karachi hotel in May that killed 14 people, including 11 French engineers.

"Asif Zaheer is an expert in explosives, and he was the man who prepared the car for suicide bombing at the Sheraton Hotel," Shah said.

Police said they had no immediate evidence of links between the foiled plot and the al Qaeda terrorist network.

The plan apparently involved loading the front of the rear-engine Volkswagen with explosives, waiting for a diplomatic vehicle to pass and ramming it to detonate the bomb.

The would-be attackers apparently decided on their plan after a June bombing outside the U.S. Consulate in Karachi killed 12 Pakistanis but no Americans.

" 'Assessing the strength of the U.S. Consulate building, we decided to target American diplomats as they traveled on city roads,' " Shah quoted Zaheer as telling police.

He said Zaheer had no apparent links to the consulate attack but wanted to drive the explosives-laden car personally to ensure the plot succeeded.

The 250 88-pound sacks of fertilizer -- not all of which would have been used in the car bombing -- could have produced a huge explosion. Shah said Zaheer told investigators it was twice the amount of explosives used in the May bombing.

Earlier today, authorities announced the arrests of three men Saturday in connection with the May suicide bombing, but it was not immediately clear if they were same three suspects discussed during the news conference.

Shah said only that six suspects are still at large in that attack.

He said Zaheer and the other suspects were trained in Afghanistan.

The militants demand that Kashmir, divided between India and Pakistan, be reunited and become either independent or part of Muslim-majority Pakistan. The Indian-controlled part of Kashmir is the majority Hindu country's only state dominated by Muslims.

Police have arrested several other people in connection with the May attack, and have apprehended dozens of others alleged to have taken part in a spate of attacks on foreigners in Karachi.

A police officer displays the Volkswagen, packed with explosives, that Islamic militants allegedly planned to ram into a diplomatic car in Karachi.