Updated

The chief investigator into the assassination of former Lebanese prime minister Rafik Hariri (search) urged Syria Tuesday to improve its cooperation with his team to help "fill in the gaps" about who planned and carried out the massive bombing in February.

Washington and Paris quickly joined forces to back a report by German prosecutor Detlev Mehlis (search) implicating top Syrian and Lebanese intelligence officials in Hariri's assassination and accusing Syria of cooperating in form but not substance with the investigation.

Mehlis, who briefed the U.N. Security Council (search) five days after the report was issued, said the extension of the investigation to Dec. 15 "would provide yet another opportunity for the Syrian authorities to show greater and meaningful cooperation, and to provide any relevant substantial evidence on the assassination."

Shortly before he spoke, U.S. Ambassador John Bolton and French Ambassador Jean-Marc de La Sabliere said they expected to circulate a joint resolution very soon. It is likely to demand Syria's cooperation with Mehlis' investigating commission, though how strong the language will be - and whether it will be under a chapter of the U.N. Charter (search) that is militarily enforceable - is still being discussed.

Mehlis' findings caused an uproar in the region and brought swift denials from the Syrian government, which called the report biased, politicized and an American plot to take over the region.

Syria's U.N. Ambassador Fayssal Mekdad told the council that every paragraph in the Mehlis report deserved to be refuted and he criticized Mehlis for accusing Damascus before the end of the investigation.

The commission "should not have pointed the finger or cast doubt on anybody," he said, adding that Syria (search) "has cooperated faithfully and sincerely with the Independent International Commission" and will continue to do so.

Boutros Assaker, the acting secretary-general in Lebanon's foreign ministry, called on "all concerned parties to sincerely and effectively cooperate" with the Mehlis inquiry to obtain "the complete and absolute truth and the punishment of all the criminals and the murderers wherever they are."

Mehlis said his team will reinterview a number of witnesses, interview new ones, complete its examination of recently obtained evidence and cooperate with Lebanese judicial and security authorities.

"The Syrian authorities may wish to carry out on their part their own investigation into the assassination of Mr. Hariri in an open an transparent manner," he said. "This would enable the commission to `fill in the gaps' and to have a clearer picture about the organizers and perpetrators of the Feb. 14 terrorist act."

Mehlis also called for stepped up security for his team of 30 investigators from 17 countries in the aftermath of the report. "The commission has received a number of threats which were deemed, in the assessment of our security personnel, to be credible," he said.

Bolton and the French ambassador - whose countries also worked together on last year's resolution demanding the withdrawal of Syrian troops from Lebanon - arrived for the Security Council meeting walking side by side.

"We will be producing a joint French-American resolution here just as soon as we can," Bolton said. "It is a matter of moving urgently, but we want to do it deliberately and prudently and with wide support, which we think we're going to get. We're very optimistic about it, but it's important to move in a careful fashion."

De La Sabliere said the two countries were working closely together on the issue.

"What Mehlis is saying is that there is that there is a lack of cooperation from Syria, and this is a very serious concern and the council should address it," he said.

After Mehlis, Mekdad and Assaker spoke, the Security Council moved into closed consultations.

The United States has intensified pressure on Syria following the report's release late Thursday: President Bush said "serious pressure" needs to be applied against Damascus but diplomacy must be given a chance before the United States takes any military action.

The Bush administration is talking about next Monday as a target date for a resolution - and a ministerial meeting of the Security Council to give its adoption added prominence. But Russia and China - both veto-wielding members of the council - don't appear in any hurry, and Moscow, which has close ties to Syria, would likely oppose sanctions or any reference to them.

France indicated Monday it would not support sanctions against Syria before Mehlis finished his investigation. Secretary of State Condoleezza Rice also has indicated the United States might be willing to put off its push for sanctions.

The Mehlis report accused key Syrian and Lebanese security officials of orchestrating the bombing that killed Hariri and 20 others. Mehlis acknowledged that he deleted references implicating the brother and brother-in-law of Syrian President Bashar Assad because he didn't know the report would be made public and the allegations were not corroborated.

The report said Syria's cooperation in form - but not substance - "impeded the investigation and made it difficult to follow leads." To complete the probe, the Syrian government must fully cooperate with investigators, including by allowing Syrians to be interviewed alone outside Syria, it said.