A top Iraqi general who switched sides during the war has backed Washington's claims that Syria has been giving refuge to members of Saddam Hussein's regime.
'The Syrian government needs to co-operate'
General Ali al-Jajjawi - former Republican Guard commander in the northern city of Mosul - said Saddam's Baath Party deputy Izzat Ibrahim and other top figures had fled to Syria shortly before the city fell last Friday.
Earlier, US President George W Bush warned Syria against harbouring fugitives from Saddam Hussein's entourage and urged Iraq's western neighbour to "co-operate" with the US-led coalition.
A senior Syrian diplomat went on American TV on Sunday to deny his country was assisting Iraqi fugitives.
We believe there are chemical weapons in Syria
US President George W Bush
Imad Moustapha, the deputy ambassador in Washington, also rejected US claims that Syria was producing weapons of mass destruction (WMD) - the charge against Iraq which sparked the war there.
"It's been a campaign of misinformation and disinformation about Syria since even before the war started - this is just an ongoing series of false accusations," Mr Moustapha told NBC news.
On the WMD charge, Mr Moustapha said: "We will not only accept the most rigid inspection regime, we will welcome it heartily."
The US has also repeatedly accused Syria of the "hostile act" of supplying Iraq with night vision goggles and other military equipment, fuelling already fraught tensions between the two nations.
The [Syrian] government is making a lot of bad mistakes, a lot of bad judgments in my view
US Defence Secretary Donald Rumsfeld
The BBC's Steve Kingstone in Washington says the warnings to Syria are being stepped up and the US is now putting down some important markers.
He says that if President Bush decides to do something more than warnings he is ready to move.
However, Britain - America's staunchest ally in the Iraq campaign - denied suggestions that Syria was "next on the list", saying there were no plans for military action against the country.
Speaking at the start of his Gulf tour in Bahrain, British Foreign Secretary Jack Straw said "there is no next list", but stressed that Syria must co-operate with the US-led coalition to answer some "important questions".
Earlier, Mr Straw told BBC Radio 4's Today programme that he was unsure whether Syria has been developing "any kind of illegal or illegitimate chemical or biological programmes".
British Foreign Office Minister Mike O'Brien was due to arrive in the Syrian capital, Damascus, later on Monday.
US intelligence has long suspected Syria of having a well-developed chemical weapons programme as well as long-range missiles.
Damascus is increasingly uneasy about the US charges
Some US experts believe Syria's programme started in earnest after clashes with Israel in 1982, with two chemical weapons plants established by 1984 to produce significant amounts of nerve gases such as Sarin and VX.
Mr Bush's comments on Sunday followed similar warnings to Syria from two of his most senior officials earlier in the day.
Defence Secretary Donald Rumsfeld, who remarked there was "no question" that Iraqi fugitives had gone to Syria, said Syrians had been discovered fighting US troops in Iraq.
"There are a number of non-Iraqis who are in the country, particularly in Baghdad we find... a lot from Syria - most from Syria, it appears," he told CBS.
"The [Syrian] government is making a lot of bad mistakes, a lot of bad judgments in my view," he added.
Ties between Syria and the US have long been strained by US support for Israel and Syria's backing of the Lebanese group Hezbollah and radical Palestinian groups, which Washington considers "terrorist".