The roads that lead from Syria to Baghdad and northern Iraq, used for years by smugglers to bring proscribed weapons to President Saddam Hussein, have now become major routes for hundreds of men wanting to become martyrs fighting against the invading U.S.-led coalition forces, according to senior administration officials.

A senior official said this week that U.S. intelligence has confirmed that the Syrian bus struck by a U.S. bomb four days into the Iraq war on a bridge near the Syrian-Iraqi border was carrying Palestinian and other volunteers into Iraq and not tourists leaving that country.

When the bus was hit March 23, it was first described as carrying Syrian tourists back to Damascus. The next day, Maj. Gen. Stanley McChrystal, vice director for operations for the Joint Chiefs of Staff, apologized for the attack and told reporters that five people were killed after the U.S. pilot launched his missile against a bridge before seeing the bus coming across it.

"It was accidental that it hit the bus, but it turned out it was a legitimate target," the senior intelligence official said. He added that it was "days after the attack" that the true purpose and destination of the passengers were discovered.

The complex and sensitive relationship between the Bush administration and the regime of Syrian President Bashar Assad has become almost byzantine in the wake of the Iraq war. A key supporter and supplier of arms to Palestinian groups, including the Hamas and Hezbollah terrorist organizations, Syria is a bitter enemy of Israel and particularly its prime minister, Ariel Sharon.

Since Sept.11, 2001, however, Damascus has been a secret partner with Washington's intelligence community in the fight against Osama bin Laden, whose fundamentalist Muslim network threatens Syria's secular government. Some information supplied by Damascus has helped break up cells in Western Europe, officials said. The relationship took on a new status in November when, at the urging of U.S. diplomats, Syria at the last minute agreed to vote for the U.N. Security Council resolution demanding that Hussein disclose and destroy his weapons of mass destruction -- making the vote unanimous.

Since that time, however, Syria has strongly criticized the U.S. invasion and continued to allow shipments to cross its border.

Late last week, as buses carrying potential fighters from Lebanon came from Syria and traveled into Baghdad, U.S. officials in Damascus repeated earlier requests that Syria shut down the traffic and close an Iraqi office that served as a front for its arms purchases.

In an interview with a Lebanese newspaper that same day, the Syrian president described his relationship with the United States as "always . . . variable." Assad said when, as with terrorism, "interests meet, the Americans are good with us, but when the interests don't meet, they want us to be courteous to them, but we don't court."

The next day, Defense Secretary Donald H. Rumsfeld in a statement warned Syria that it would be held "accountable" for shipment of military supplies to Iraq via Syria. Although Rumsfeld singled out "night vision goggles" in his warning to Syria, he said, "These deliveries pose a direct threat to the lives of coalition forces." Rumsfeld also warned Iran to keep armed Iraqi exile groups based in Tehran from returning to Iraq as they had done in 1991.

"What Rumsfeld did was basically to deliver a shot across the bow of Syria and Iran in the heat of battle in Iraq," said Edward P. Djerejian, a former U.S. ambassador to Damascus who is now director of the Baker Institute for Public Policy at Rice University. "He was looking at the progress of the war plan and expressed concern and nervousness about what was happening across the border."

Djerejian described Rumsfeld's statement as "a preemptive public diplomacy strike at both countries" and not as a prelude to "what's next after Iraq."

Secretary of State Colin L. Powell used a speech before the American Israel Political Action Committee, a strongly pro-Israel group, to reinforce Rumsfeld's charges, saying, "Syria can continue direct support for terrorist groups and the dying regime of Saddam Hussein, or it can embark on a different and more hopeful course."

Syrian officials took the remarks of both officials seriously. Syria's ambassador to the United States, Rostom Zoubi, described Rumsfeld's charges as "untrue and unfair" on Larry King's CNN program March 30, and he linked them to the same accusations in an Israeli newspaper the same morning. Assad on Wednesday told an Austrian newspaper that Powell showed a "lack of foresight" in threatening Syria.

Behind the rhetoric from Damascus is concern that top officials in the Bush administration have targeted Syria as the next country for a change in government. In 1996, Douglas Feith, now undersecretary of defense for policy and an architect of plans for a post-Hussein Iraq, co-authored an advisory paper with Richard Perle, a close Rumsfeld adviser, for then-Israeli Prime Minister Benjamin Netanyahu that called for a "clean break" from the traditional peace process approach between the Arabs and Israel. They advised replacing it with a stronger military approach and confrontation with Iraq and Syria for their support of terrorists.

This week, Perle wrote during an online exchange just published by Foreign Policy magazine: "Would you rather talk with Syrian President Bashar al-Assad about terrorism before or after the liberation of Iraq?"