US Secretary of State Colin Powell has issued a fresh warning to Iraq's neighbours, Syria and Iran, to stop supporting terrorism.
Mr Powell's warning followed one from Donald Rumsfeld on Friday
Widening the range of US concerns beyond Iraq, Mr Powell said Syria must abandon its "direct support for terrorist groups".
He also said it was time for "the entire international community to insist that Iran end its support for terrorism".
Mr Powell's comments came two days after US Defence Secretary Donald Rumsfeld warned both countries not to get involved in the conflict in Iraq.
The Syrian Information Minister, Adnan Umran, dismissed Washington's accusations as false.
Syria bears the responsibility for its actions and for their consequences
US Secretary of State Colin Powell
In an interview with the BBC Arabic Service, Mr Umran denied charges that Arab volunteers going to fight in Iraq had passed through Syrian territory.
Mr Powell was speaking to the American Israel Public Affairs Committee - the most powerful pro-Israel lobbying group in the United States.
Mr Powell said: "Syria can continue direct support for terrorist groups in the dying regime of Saddam Hussein or it can embark on a different and more hopeful course.
"Either way Syria bears the responsibility for its actions and for their consequences."
He said that Iran must also stop its pursuit of weapons of mass destruction and the ability to produce them.
Some Syrians believe the US wants other regime changes
But Mr Umran pointed to the fact that Mr Powell's remarks were made at the meeting of a Zionist organisation which, said the minister, had absolute influence over the Bush administration.
The BBC's Kim Ghattas in Damascus says Syria will probably not deny what Mr Powell has said about support for organisations such as Hezbollah.
"They will say they are supporting those organisations offering legitimate resistance to Israel," she says.
Our correspondent says that in any case, it now looks as if relations between Syria and the US are at an all-time low.
In Iran on Monday, one of the leading reformist clerics, Ayatollah Ali Montazeri, condemned all the countries co-operating with the US-led coalition, naming Kuwait and Turkey.
"The people of the region will never forget that they served the cause of world oppression," Ayatollah Montazeri said.
About 400 Syrian women protested in central Damascus on Monday, shouting anti-US slogans.
Hours before Mr Powell spoke, Syrian Foreign Minister Farouk al-Sharaa told parliament Syria had "a national interest in the expulsion of the invaders from Iraq".
On Friday, Mr Rumsfeld said the US believed that military equipment, including night-vision goggles, had passed through Syria to Iraq.
Syria said the US was trying to justify the failures of its military campaign.
Last Thursday, an interview appeared in the Lebanese newspaper, as-Safir, in which Syrian President Bashar al-Assad said it was a possibility that Syria would be next on Washington's list.
Our correspondent says there is a strong sense in Syria that the US "wants to redraw the map of the region and change other regimes as well".
In Egypt's al-Ahrar newspaper on Monday, Arab League Secretary-General Amr Moussa was quoted as saying that the allegations that military supplies were entering Iraq from Syria would increase the volatility of the regional situation.