Nairobi, New York, Bali. And now Riyadh.

Months, even years can go by between carefully planned Al Qaeda (search) attacks, often leaving the mistaken impression that Usama bin Laden's (search) organization is becoming too weak to strike. But each attack serves as a reminder of the terrorists' reach and their influence on other militant groups worldwide.

While the United States has made significant progress in fighting Al Qaeda in Pakistan and Yemen, where the FBI and CIA are operating with local authorities, the battle has been far more difficult in places such as Iran (search) and Saudi Arabia (search), where U.S. authorities have been unwelcome.

Still, initial intelligence assessments point to an outside command-and-control structure that directed the Saudi operation, according to intelligence sources who spoke on condition of anonymity.

It was unclear whether Al Qaeda operatives who have floated into Iran from Afghanistan (search) in recent months may have been involved.

"We know there is senior Al Qaeda in Iran," Secretary of Defense Donald H. Rumsfeld said Thursday.

U.S. officials have identified three Al Qaeda leaders in Iran: Saif al-Adil (search), bin Laden's security and intelligence chief; Saad bin Laden (search), Usama's son; and Abu Hafs the Mauritanian (search), a religious scholar.

To what extent the Tehran government tolerates their presence is not known, U.S. officials say. But Rumsfeld identified Iran as a country that is harboring Al Qaeda leaders.

A senior Turkish intelligence official recently told The Associated Press that the Iranians have been sending planeloads of low-level Al Qaeda fighters back to Saudi Arabia, but that senior figures are trying to stay in Iran.

Abu Musab Zarqawi (search), a senior associate of bin Laden, passed through Iran after fleeing Afghanistan. But he went to Baghdad for medical treatment — and became a key factor in the U.S. allegations that Saddam Hussein's government had contacts with Al Qaeda. His whereabouts are unknown.

Bin Laden and his chief deputy, Ayman al-Zawahri, are still thought to be in the mountainous region along the Afghan-Pakistani border. Rumsfeld said Thursday that the United States still doesn't know whether the Al Qaeda chief is alive.

Rumsfeld emphasized that 18 months of battling Al Qaeda had led to significant gains, including a crippling of the network's communications and elimination of Afghanistan as a base.

But experts said Monday's attack in Saudi Arabia and others in recent months show Al Qaeda can carry out successful operations on the move.

"Terrorist organizations don't need bases anymore," said Zvi Barel, a Middle East analyst for the Israeli newspaper Ha'aretz. "They can operate from anywhere. They have the Internet, they can buy explosives and weapons in any country."

Barel said Al Qaeda's success over the years buys it influence with other militants.

"Groups in Algeria, Yemen and Lebanon have their own reasons to exist. But at the same time, they are able to provide services, like subcontractors, to whoever wants to pay."

Most of the recent successes in the fight against Al Qaeda have come in Pakistan. In March, Pakistani and CIA operatives captured suspected Sept. 11 mastermind Khalid Shaikh Mohammed near Islamabad. A financier was detained in the same raid.

Later that month, a bin Laden messenger was taken in Lahore. Al Qaeda training chief Abu Zubaydah was captured last year in Faisalabad.

In Yemen, scene of several Al Qaeda attacks, the CIA killed the group's top operative in the country. However, two key planners of the 2000 bombing of the destroyer USS Cole recently escaped from prison there and remain at large.

But in Saudi Arabia, where successive U.S. administrations have maintained close relations with the kingdom's oil-rich royals, polls and appearances indicate anti-Americanism and support for bin Laden has been growing. At the same time, Saudi cooperation in fighting terrorism has not improved — although that could change after this week's deadly attacks; an FBI team is already in the country.

"Saudi Arabia and the United States have worked together and need to work together on a continuing basis and an improving basis to exchange information about how we can devalue the threat or destabilize or disrupt terrorist activity," said Attorney General John Ashcroft.

The Bush administration announced at the end of the Iraq fighting that it would remove most of its military from Saudi Arabia. Since its beginnings, Al Qaeda has demanded the United States leave the Muslim holy land.

The Saudi attacks prompted Britain's Department of Transport on Thursday to cancel all British flights to and from Kenya because of an increased terrorist threat in the African country.

In November, a missile attack on a hotel frequented by Israeli tourists to Mombasa, Kenya, killed 11 Kenyans and three Israelis. The attack, which included a failed attempt to shoot down an Israeli plane, was thought to have been the work of Al Qaeda.

Police in Kenya said Thursday that an Al Qaeda suspect wanted in the 1998 U.S. Embassy bombing in Kenya and the Nov. 28 attacks on the Kenyan coast had returned to the country. Fazul Abdullah Mohammed could be planning another attack, said Matthew Kabetu, head of Kenya's anti-terrorism unit.

Sen. Bob Graham suggested this week that the Iraq war may have hurt the war on terrorism.

"I would say that Al Qaeda had substantially weakened and was on the ropes about 12 to 14 months ago," he said. "As I said last fall, I thought the priority for the United States should be to win the war on terror before we took on the other evils in the Middle East and Central Asia."