All options are open

"Our patience is running out with Syria," Khalilzad told a press conference. When asked how the United States could respond, he said "all options are on the table", including military.

"I would not like to elaborate more, they should understand what I mean," he added.

The United States believes that Iraqi Sunnis are refusing to agree a proposed constitution because of threats from Sunni extremists who have infiltrated Iraq from Syria, where they have training camps, Khalilzad said.

"Syria has to decide what price it's willing to pay in making Iraq success difficult. And time is running out for Damascus to decide on this issue," the ambassador warned.

"It simply must close the training camps," he said. "It should not allow youngsters misguided by al-Qaeda, from Saudi Arabia, from Yemen, from North Africa, to fly into Damascus international airport."

"It shouldn't be that hard, if you see young men between the ages of 18 and 28, who are coming without a return ticket, landing in Damascus airport to control that."

The former US ambassador to Kabul also warned that Iraq could become worse that Afghanistan under the Islamic fundamentalist Taliban if extremists like Abu Musab al-Zarqawi, al-Qaeda's frontman in Iraq, were to prevail.

"If people like Zarqawi were to dominate Iraq, it will make Afghanistan under the Taliban look like a picnic, given the resources of Iraq, the location of Iraq," Khalilzad said.

Khalilzad said he had accompanied Iraqi President Jalal Talabani to Washington. Talabani will meet President George W. Bush on Tuesday before heading to New York for a summit of world leaders at the United Nations.

Khalilzad, who is the former US ambassador to Afghanistan, compared the war on terrorism to the Cold War and World War II.

"As during the Soviet era, Soviet communism was the defining of our time, now it's terrorism and extremism that's the defining challenge of our time," he said.

Meanwhile, Syria rejected on Monday US accusations that it allows extremists to sneak into Iraq, saying Washington's "threat" of using force was part of relentless pressure on Damascus.