In addition, he said Syria continues to support organizations the United States lists as terrorist groups.
John Bolton, undersecretary of state for arms control, told a House hearing the United States was trying to change Syria's behavior through diplomatic means and urged lawmakers to let the effort run its course before passing trade restrictions and other measures.
After testifying to the House International Relations Committee's panel on the Middle East and Central Asia, Bolton left for Moscow, where he is to talk with the Russians about proliferation of nuclear technology in Iran, State Department deputy spokesman Adam Ereli said.
White House national security adviser Condoleezza Rice said on ABC's "Nightline" that "Syria is a country with which we continue to have a number of problems."
"We don't really feel they're meeting the mark, but we continue to press the Syrians," she said.
Secretary of State Colin Powell said Monday in Kuwait that Syria was not cooperating with U.S. demands to end support for the Iranian-backed Hezbollah group, which Washington labels a terrorist organization, and to withdraw its troops from Lebanon. He warned Congress would adopt the legislation if Syrian President Bashar Assad did not act.
Syria's foreign minister, Farouk al-Shaara, rejected charges his country was not cooperating and said in Damascus that Syria was willing to meet "reasonable" demands within the framework of international legitimacy.
"What Powell said about Syria's cooperation, I ask: Who is cooperating with America as America wants?" al-Shaara said at a news conference. "America has too many demands. If they are reasonable and realistic Syria is ready to cooperate."
Bolton contended at the hearing that Syria "permitted volunteers to pass into Iraq to attack and kill our service members during the war and is still doing so."
Defense Secretary Donald H. Rumsfeld said in Iraq last week that the largest numbers of figures captured in Iraq have been from Syria, then Lebanon. Defense officials said there have been about 200 foreign fighters rounded up so far in Iraq.
Rice said that the estimates of the number of foreign terrorists filtering into Iraq range from "several hundred — high hundreds — to a couple of thousands." She suggested that the majority of those individuals were among the 20,000 to 30,000 — or "possibly even a bit higher" — who passed through al-Qaida training camps.
"These are hard-core training terrorists," she said. "Are they actually being sent by al-Qaida or are they, in a sense, freelancing? I think we don't know the full picture. But these are people who were committed jihadists, who were part of the war on terror."
Bolton, who testified first in public and then in a classified session with lawmakers, said Syria offers sanctuary and political protection to groups such as Hezbollah, Hamas and Palestinian Islamic Jihad — all branded as terrorist by the United States.
He said while there is no information Syria has transferred weapons of mass destruction to the terrorist organizations, "Syria's ties to numerous terrorist groups underlie the reasons for our continued anxiety."
Bolton put Syria in the same category as Iran, North Korea and Libya 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."
He said the U.S. objective was not just to prevent the spread of these weapons but to "roll back and eliminate such weapons from the arsenals of rogue states" through peaceful and diplomatic means. However, he continued, in order to do this and protect America and its allies, "we must allow ourselves the option to use every tool in our nonproliferation toolbox."
Asked by the subcommittee's ranking Democrat, Rep. Gary Ackerman, D-N.Y., if this would include regime change in Syria, Bolton replied the administration preferred diplomacy "but I am not taking any option off the table."
In her opening remarks, the committee chairman, Rep. Ileana Ros-Lehtinen, detailed Syria's unconventional weapons capability, quoting from an unclassified CIA report to Congress that was issued in April and covers the first six months of 2002.
The report stated that "Syria sought chemical weapons-related precursors and expertise from foreign sources, maintains a stockpile of the nerve agent sarin and appears to be trying to develop more toxic and persistent nerve agents."
She said Syria's recent agreement with Russia concerning close cooperation on nuclear power "raises grave questions regarding the Syrian regime's true objectives on the nuclear front."
Ros-Lehtinen, R-Fla., said in an interview after the hearing she would like to see the Bush administration take a stronger stand on Syria. She is co-sponsoring a bill that could lead to sanctions against Syria.
Appeasing "these terrorist regimes does no good to the United States," she said.