The US Congress has begun debating whether to impose sanctions on Syria.
Powell: Syrians have not complied with US wishes
It is examining proposed legislation that accuses Damascus of supporting terrorism and developing weapons of mass destruction, and condemns its military presence in Lebanon.
Known as the Syria Accountability and Lebanese Sovereignty Restoration Act, the legislation would require Damascus to change its behaviour or face American sanctions.
Although the Bush administration has not taken position on the proposed act, it has recently hinted at a tougher US stance towards Syria.
During a visit to Damascus following the US-led war in neighbouring Iraq, US Secretary of State Colin Powell issued a warning to the country's leadership.
He told it not to give refuge to members of Saddam Hussein's former regime or allow volunteers to cross from its territory onto Iraqi soil to fight the Americans.
On Monday Mr Powell said Damascus had fallen short of the required co-operation.
"So far the Syrian leadership have not responded as
forcefully... as I would have liked, thus the Congress is debating
the act," he said.
On Tuesday, US undersecretary of state for arms control John Bolton told a congressional panel that Syria was a security concern on two accounts - its alleged pursuit of weapons of mass destruction and its record of supporting militant groups.
Mr Bolton said that while Washington would continue to use diplomacy to make its case, it should not rule out "more robust" means - such as sanctions.
BBC Middle East analyst Roger Hardy says the Bush administration has not so far supported the Syria Accountability Act because Mr Powell feels that, if passed, it would tie his hands.
But he adds that the administration is not averse to using the threat of sanctions as an instrument of persuasion.
Syria denies harbouring terror groups and has promised to meet any "reasonable" request from the United States for help.
Its president, Bashar al-Assad, appointed a new prime minister last week in the first such change since he succeeded his father as president in 2000.