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 Rafik Hariri, according to a copy of a letter obtained Tuesday by the Washington Post 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 UN investigation into Hariri's death. Late Tuesday, the US, France and Britain challenged the rest of the UN Security Council to adopt a very tough resolution against Syria which would threaten sanctions if Damascus fails to cooperate fully with the UN investigation. "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. According to the copy of the letter provided by diplomatic sources in Damascus, "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." The US 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 UN 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." 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. The tough draft resolution submitted by the US, UK and France demands Damascus detain possible suspects and make them available to UN investigators, who have complained about Syria's cooperation. If Syria does not do this, the text says, the council would consider "further measures," such as economic sanctions, "to ensure compliance." The pressure on Syria is likely to intensify Wednesday when a report by the UN special envoy on Syria-Lebanon, Terje Roed-Larsen, on disarming Lebanese militias is released. There are allegations that Syria is continuing to smuggle arms to Palestinian militia groups in Lebanese refugee camps, in violation of a council resolution adopted in September 2004 demanding that all militias be disarmed.