Under mounting international pressure, Syrian President Bashar Assad has promised that any Syrian accused will face trial if "proved by concrete evidence" to have had a role in the killing of former Lebanese prime minister Rafiq Hariri, according to a copy of a letter obtained Tuesday and sent to the United States, Britain and France.

The pledge, in a letter from Assad dated Sunday, is the most substantive response by Syria to the U.N. investigation into Hariri's death, which concluded that senior Syrian officials almost certainly had a role in the Feb. 14 car-bomb assassination in Beirut. Mid-level officials here have sought to dismiss the report as unprofessional and politicized, although Assad has not commented publicly. The letter denied any Syrian role in the killing, but suggested the government would cooperate to deflect criticism.

The U.N. Security Council began discussing the investigation on Tuesday, and U.S., French and British diplomats circulated a draft resolution threatening Syria with sanctions if it fails to cooperate with the continuing probe.

"I have declared that Syria is innocent of this crime, and I am ready to follow up action to bring to trial any Syrian who could be proved by concrete evidence to have had connection with this crime," Assad said in the letter.

He warned against using the U.N. report as a political tool to pressure Syria. The United States has criticized the Syrian government for supporting radical Palestinian factions, intervening in Lebanon and failing to seal its border to foreign fighters infiltrating Iraq.

"Such use of this report will have big, serious repercussions on the already tense situation which our region goes through, at a time we are more in need to have objective and constructive positions that would help the countries of the region to achieve stability," said the copy of the letter provided by diplomatic sources in Damascus.

The U.S. State Department expressed skepticism about the new offer. "Once again Syria has demonstrated by its policies and its actions that it's out of step with the international community and in this instance specifically, by its failure to correctly read the tea leaves and fully cooperate" with the U.N. investigation, State Department spokesman J. Adam Ereli told reporters Tuesday.

"That is why you have a Security Council that's meeting to come to some conclusion about what to do about that failure to cooperate," Ereli said. "So it's a little late now for Syria to try to be making up for past failures."

The U.N. investigation, headed by German prosecutor Detlev Mehlis, and the international criticism of Syria it provoked have created the greatest crisis for Assad since he succeeded his father, Hafez Assad, who died in 2000. The Syrian government, increasingly isolated both in the Middle East and abroad, was believed to have been caught off-guard by the sweep of the report, which names officials representing a cross-section of Assad's government. Among those mentioned are the president's powerful brother-in-law, Assef Shawkat, a general who heads Syrian military intelligence, but Mehlis stressed that no one had yet been proven guilty.

The degree of Syria's cooperation with the U.N. investigation probably will determine whether the country faces punitive sanctions, diplomats said. Splits have already emerged among permanent members of the Security Council, with Russia and China saying it is premature to discuss sanctions before Mehlis concludes his probe on Dec. 15. Analysts and diplomats here expect Syria to cooperate, although to what degree is unclear. In the past, the government has adhered to a reactive strategy of offering some concessions, in an attempt to weather any storm, while retaining trump cards that officials believe they can bargain with.

Unlike in past years, however, the United States has severed all but routine diplomatic contacts with the Syrian government and rebuffed attempts in recent months to resume dialogue on intelligence issues. Even fellow Arab countries, in particular Egypt and Saudi Arabia, have demonstrated almost no public willingness to mediate on Syria's behalf.

"I'm sure they can do enough to avoid sanctions, but how do you get beyond that?" a diplomat in Damascus said, speaking on condition of anonymity. "It's not going to get them off the hook."

The letter does not specify whether Syrians accused in the case would be prosecuted in a Syrian or foreign court or what would constitute "concrete evidence." A crucial test ahead may be whether Syria permits accused officials to be questioned abroad, a move that might encourage Syrian officials to strike a deal but that many analysts and diplomats do not deem especially risky for the government.

"They'll say the same story," the diplomat said. "They're professionals. I don't think it will actually change a huge amount."

There were at least two slightly different versions of Assad's letter, diplomats said. The one with the pledge to bring to trial any Syrian implicated in Hariri's assassination was delivered to the United States, Britain and France, among others; another version omitting the pledge went to other Security Council members, the diplomats said.

Staff writer Robin Wright in Washington contributed to this report.

College students in Damascus wave Syrian flags during a protest against a U.N. report implicating government officials in the killing of a former Lebanese prime minister.The U.N. probe led by German prosecutor Detlev Mehlis has created the biggest crisis of Bashar Assad's presidency.