The United States hoped Bashar Assad (search) would show more flexibility on the Middle East conflict than his father had.

Yet despite some behind-the-scenes Syrian help in the war on terror, America's list of grievances remains long against Assad, who became his country's leader three years ago.

As the Israeli-Palestinian situation deteriorates, Israel's weekend airstrike against a purported Palestinian terrorist training camp inside Syria (search) could escalate tensions once again.

U.S. officials used the occasion to highlight their concerns about Syria's ties to Palestinian militants.

"We've seen Syria as a state sponsor of terrorism for a long time," said State Department spokesman Richard Boucher (search). "We've repeatedly made known our grave concerns about Syrian support for terrorist groups, including Palestinian groups."

U.S. officials also are worried about Syria's possession of unconventional weapons and its support for the Lebanon-based Hezbollah (search) guerrillas. Another sore point for Washington is Syria's purported role in allowing activists to cross into Iraq to take up arms against U.S. soldiers.

The weekend raid was in retaliation for a homicide bombing that killed 19 people, and President Bush on Monday spoke up for Israel's right to defend itself.

In general, U.S. officials seem pleased that Assad's government is content for the time being to deal with the issue at the U.N. Security Council, where Syria is calling for a condemnation of the Israeli strike. The United States has made clear it opposes that U.N. effort, and Syria has accused the United States of opening the way for more Israeli attacks.

The situation poses the most serious foreign policy challenge Assad has faced since he assumed command in June 2000 after the death of his father, longtime hardline Syrian President Hafez Assad.

Officials said there were hopes that the younger Assad would chart a different course.

But Richard Murphy, a former assistant secretary of state, said the younger Assad was put into power by his father's supporters.

"Whatever he may have wanted to do, he was beholden to those supporters," Murphy said.

Assad is more soft-spoken than his father and perhaps more worldly, based on an education in the West, said James Philips, a Middle East expert at the Heritage Foundation. But Phillips said Assad and his government seem to lack the ability to adapt to changing circumstances.

Ammar Alarasan, a spokesman at the Syrian Embassy, said it is America's policy, not Syria's that suffers from inertia.

"The United States still doesn't understand the region and why there are reactions to its policies," Alarasan said.

He added that Syria and other Arab countries joined the United States in the fight against terrorism after the Sept. 11 attacks.

"We acted on the principle that Syria is against fundamentalism and terrorism," Alarasan said.

Indeed, U.S. officials acknowledge Syria's counterterrorism cooperation.

They note that authorities in Damascus have in custody Mohammed Haydar Zammar, who is believed to have recruited Mohamed Atta and two other Sept. 11 hijackers while they lived in Germany in the late 1990s.

After the attacks, Zammar, a Syrian by birth, was captured in Morocco. U.S. intelligence has access to reports from his interrogations, although it is unclear if the Syrians provide them directly or the United States obtains them through an intermediary.

In addition, some high-level Iraqis tracked to Syria after the war in Iraq later turned up in U.S. custody, prompting speculation the Syrians expelled them under U.S. pressure. They include Farouk Hijazi, a former high-level official in the Iraqi intelligence service.

Despite such behind-the-scenes assistance, U.S. officials are concerned about Syrian efforts to expand unconventional weapons capabilities. Government experts believe Syria has possessed the nerve agent sarin for some time and has been trying to develop more toxic nerve agents.

U.S. Undersecretary of State John Bolton testified last month that Syria is seeking biological and nuclear weapons as well.

Bolton put Syria in the same category as Iran, North Korea and Libya, describing them as "rogue states, those most aggressively seeking to acquire weapons of mass destruction and their delivery -- and which are therefore threats to our national security."