The string of blasts occurred in quick succession, the last coming early Tuesday outside the headquarters of a joint U.S.-Saudi owned company. 

"We dont know how many are injured, but we received 50 and the number is growing," an official at the National Guard Hospital in Riyadh told The Associated Press by telephone, without identifying himself. "We're very busy, we are receiving a lot of casualties." 

Three Western residential compounds were attacked, an American who lives in one of the targeted areas told the AP in an e-mail exchange from Riyadh. There was extensive damage to property, he said on condition of anonymity, adding that he believed there had been some deaths. 

A U.S. official traveling with Secretary of State Colin Powell said they had been told that there were no American casualties in the explosions. 

Powell, who is currently in neighboring Jordan, will go to Saudi Arabia on Tuesday as scheduled, said the official, speaking on condition of anonymity. He is seeking the Saudis' help in harnessing militant groups and in promoting Palestinian reform in the latest stop on a Mideast tour that has already taken him to Israel, the West Bank and Egypt. 

Initial U.S. suspicion for the attacks centered on the al-Qaida terror network, a U.S. counterterrorism official in Washington said on condition of anonymity. Intelligence from the past two weeks indicated the terrorist organization Qaida was close to launching a strike in Saudi Arabia, the official said. The State Department had advised Americans earlier his month against travel to Saudi Arabia because of increased terrorism concerns. 

Saudi officials have recently announced that it foiled planned al-Qaida attacks in the oil-rich kingdom, which is the birthplace of Osama bin Laden. 

A Saudi security official told The Associated Press that a black Chevrolet Caprice sedan crashed into a residential compound in Garnata, an eastern suburb in Riyadh. The officials said the explosion caused a number of injuries. 

The names of the other two Western compounds attacked were not immediately known. 

Witnesses told the AP that the force of the blast in Garnata shook nearby buildings and rattled windows. Witnesses also reported hearing gunfire moments before the car exploded. 

Smoke lingered over the compound as police cars and ambulances rushed in. Hundreds of anti-riot police and members of the elite National Guard converged on the scene, evacuating compound residents and sealing off the area. The compound is owned by Riyadh's deputy governor Abdullah al-Blaidh and includes several residential complexes housing mainly Westerners and non-Saudis. 

The fourth blast went off at the headquarters of the Saudi Maintenance Company, also known as Siyanco, early Tuesday morning. 

The company is a joint-owned venture between Frank E. Basil, Inc., of Washington, and local Saudi partners, the officials reported. 

Defense Secretary Donald Rumsfeld announced last week that most of the 5,000 U.S. troops in Saudi Arabia would leave by the end of the summer. The presence of U.S. troops has been a major irritant to the kingdom's rulers, who face strong anti-American sentiment from the population. 

The American military presence in Saudi Arabia, birthplace of Islam, was also among the reasons used by bin Laden as his rallying call for Muslims to attack U.S. interests worldwide. The kingdom was home to 15 of the 19 Sept. 11, 2001 attack hijackers. 

Last week, a senior Saudi security official said suspected terrorists were receiving orders directly from bin Laden and were planning attacks in Saudi Arabia targeting the royal family as well as American and British interests. 

The official, who spoke on condition of anonymity, said the prime targets were the defense minister, Prince Sultan, and his brother, the interior minister, Prince Nayef. 

On Wednesday, authorities said they foiled plans by at least 19 suspected terrorists to carry out strikes and seized a large cache of weapons and explosives in the capital. 

All escaped after a gunfight with police. 

In remarks published Thursday, Prince Nayef said the men could be linked to bin Laden's al-Qaida terrorist network, which now was "weak and almost nonexistent." 

Nayef said the men included 17 Saudis, an Iraqi holding Kuwaiti and Canadian citizenship, and a Yemeni. "These men have only one goal in mind: Jihad (holy war) ... They have been brainwashed," he said. 

Their names and pictures were shown on state-run Saudi television Wednesday, and a reward of more than $50,000 has been offered to anyone turning in any of the suspects. 

The confiscated weapons included hand grenades, five suitcases of explosives, rifles and ammunition, as well as computers, communications equipment and cash, officials said. 

News of the plot came a week after an American civilian working for the Saudi Royal Navy was attacked and slightly injured in eastern Saudi Arabia. 

In 1996, a truck bombing killed 19 Americans at the Khobar Towers barracks in Dhahran. In 1995, a car bomb exploded at a U.S.-run military training facility in Riyadh. Seven people died, including five American advisers to the Saudi National Guard. The Islamic Movement for Change and two smaller groups in the region claimed responsibility.