By Justin Webb
BBC correspondent in Washington
The Pentagon has warned Syria of the risks it faces if foreign fighters continue to use its territory to get into Iraq.
Syria's President Bashar al-Assad has been under US pressure
The warning comes after fierce fighting near Iraq's border with Syria over the weekend.
In a separate development, President Bush's Democratic party presidential challenger, John Kerry, has sharpened his attacks on the President's handling of the situation in Iraq.
The latest warning to Syria from the Bush administration was delivered by America's most senior soldier, the chairman of the Joint Chiefs of Staff, General Richard Myers.
He said America knew that the pathway into Iraq for many foreign fighters was through Syria.
"It's a fact," said Gen Myers. "We know it, the Syrians know it.
"The Syrians need to take this situation very seriously," he went on, "it doesn't do their government any good."
Gen Myers seemed to be suggesting that the foreign fighters might harm Syria through their destabilisation of the region.
He was not suggesting that America had any hostile action in mind.
As he considers his next moves in Iraq, President Bush is having to cope with a daily onslaught from his political opponents here in America.
Senator Kerry is now openly using the Iraq situation to try to persuade the electors to ditch the president in November.
The senator said America needed a new president to re-establish its credibility with the rest of the world.