"We now undertsand that Syria has closed its borders to all but humanitarian traffic," US State Department spokesman Richard Boucher said. "That's what they have told us and we certainly hope that proves to be true."

The US ambassador to Syria, Theodore Khattouf, had been told of the closure in meetings with Syrian officials in Damascus but stressed that Washington would be watching the border closely to see if the move was enforced, the State Department said.

Boucher said the US military and intelligence agencies would be monitoring the frontier "quite closely" and he repeated warnings issued over the past few days by senior US officials that Syria faced a critical choice in its dealings with Iraq.

"Syria has choice to make and we hope Syria makes the right one," Boucher told reporters.

A senior State Department official said later it was possible that Damascus's definition of "humanitarian traffic" might well be different than Washington's.

The comments came amid a flurry of allegations from Defense Secretary Donald Rumsfeld and others that Syria is allowing military equipment and irregular troops to cross its border into Iraq to help Baghdad.

In addition, there have been accusations that Syria has allowed members of Saddam's regime to cross into its territory to seek safe haven.

Deputy Defense Secretary Paul Wolfowitz told lawmakers Thursday that Syria had been "behaving badly" but that no US military intervention was anticipated.

"The Syrians are behaving badly, they need to be reminded of that and if they continue we need to think about our what our policy is," Wolfowitz said in testimony to the Senate Armed Services Committee.

"It's very dubious behaviour, and by calling attention to it we hope that in fact it will be enough to have them stop," he said, adding that Syria harbours terrorists and war criminals and has shipped "things" to Iraq.

Wolfowitz's testimony followed remarks by Secretary of State Colin Powell in an interview published Thursday in which he sought to assuage Syrian and Iranian fears that they might be the next targets of US military action.

"We believe that all of these nations -- Syria, Iran, others -- should realize that pursuing weapons of mass destruction, supporting terrorist activities, is not in their interest," Powell told the Los Angeles Times.

"That doesn't mean that war is coming to them, it just means that the world is changing," he added.

Both Syria and Iran, along with Iraq, North Korea and three other nations, are designated as "state sponsors of terrorism" by the United States which also accuses them of pursuing weapons of mass destruction.

Defense Secretary Donald Rumsfeld said on Wednesday that Syria seemed to be ignoring Washington's increasingly strident warnings against providing military assistance to Iraq.

"They seem to have made a conscious decision to ignore that. We find it notably unhelpful," Rumsfeld said.

He did not elaborate on which members of the Iraqi leadership may have slipped into Syria, but a senior US official said there were strong indications that several of Saddam's relatives, including his first wife, had crossed the border in the days leading up to the war.

The same offical said that others believed to have ties to Saddam, although not necessarily high up in the government, were thought to have gone into hiding in Syria more recently.

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