The U.N. Security Council has voted unanimously to extend the investigation into the assassination of former Lebanese Prime Minister Rafik Hariri. But the Council turned down a Lebanese request to broaden the probe.

A resolution adopted late Thursday gives investigators another six months to unravel the mystery surrounding the Hariri killing. It criticizes Syria's lack of cooperation with the investigators led by German prosecutor Detlev Mehlis.

Washington's U.N. Ambassador John Bolton hailed passage of the measure, saying it puts additional pressure on Syrian officials suspected of involvement in the assassination. "The United States believes it sends a strong signal to Syria that we still require full and unconditional compliance with obligations of earlier resolutions to cooperate with the Mehlis commission. It's clear they have not yet provided that cooperation," he said.

But the resolution was approved after a long day of negotiation in which two requests made by Lebanon were effectively denied. Lebanese leaders had asked that the investigation be broadened to include other political assassinations, and that suspects would be tried by an international tribunal.

Instead, the resolution offers technical assistance to Lebanese investigators. ///START OPT/// Co-sponsors the United States, Britain and France had favored expanding the probe. But afterward, Ambassador Bolton said he was not disappointed by the outcome. "No, I don't think it's a bitter pill (setback). This is the Security Council. A number of delegations said they wanted a unanimous resolution. They had certain changes they had to be able to join the unanimous decision. We did not feel it crossed any of our red lines. We would have won a vote had we put it to a vote, but we considered that unanimity had a certain value as well. So a unanimous decision is not a bitter pill," he said.

Approval of the resolution came days after the Mehlis Commission issued a report saying new evidence in the Hariri case had reinforced its conclusion that Syrian and Lebanese intelligence officials were involved in the murder.

Syria has strongly denied having had a role in the killing, and after the Council's vote, Syria's U.N. Ambassador Fayssal Mekdad expressed satisfaction that the language of the resolution had been softened. He said Damascus had nothing to hide and would cooperate with investigators. "There was even no need for certain paragraphs. Because we feel it is our responsibility and self-imposed duty to cooperate fully with the commission. You will ask me why. And the question is very simple. Because we are fully confident that Syria is innocent. Syria will never be behind such heinous actions and crimes. We condemned all these crimes, and this has never been our policy," he said.

While the mandate of the investigation will continue through next June, German prosecutor Detlev Mehlis is returning to his old job in Berlin.

In a CNN interview, Mr. Mehlis said he sees a clear link between the Hariri assassination and the other recent political killings in Lebanon.

Secretary-General Kofi Annan has admitted having difficulty finding someone to take over the high-pressure job, and declined to predict when he might name a successor.