"We expect to introduce a resolution before the end of this week," US Ambassador to the UN Zalmay Khalilzad told reporters, adding it was in response to a request from the "legitimate government" of Lebanese Prime Minister Fouad Siniora.

On Monday, Siniora sent a letter to UN chief Ban Ki-moon asking him "as a matter of urgency to put before the Security Council our request that the special tribunal be put into effect" to try suspects in the 2005 murder of Hariri.

Following a luncheon meeting with Security Council ambassadors Tuesday, Ban endorsed plans by the United States, Britain and France to introduce a draft under Chapter Seven of the UN Charter that would effectively impose the creation of the tribunal.

"I think it is necessary for the Security Council to take the necessary action," the UN chief told reporters. "I am of the view that there should be no impunity for the perpetrators of political assassinations."

Siniora's Western-backed government has been locked in a bitter feud with its pro-Syrian opposition over implementing an agreement with the UN to set up the contentious court.

Hariri and 22 other people were killed in a massive bomb blast in February 2005, widely blamed on Syria, which was then forced to end nearly 30 years of military and political domination in Lebanon.

Siniora pressed for a binding Security Council resolution because his pro-Syrian opposition has been blocking parliamentary ratification of the tribunal plan, saying the UN would use it for political ends.

Speaking in his capacity as US envoy, Khalilzad, who chairs the 15-member Security Council this month, told reporters Tuesday that it was "important to assist the Lebanese" in setting up the tribunal.

"We cannot let the Lebanese down. It is imperative that we move forward...for the long-term stability of Lebanon," he added.

"Of course it would have been desirable that this could have done internally ...We believe we need to respond move and and respond expeditiously. We cannot allow killers to get away with impunity," he told reporters.

Diplomats said the text would be sponsored by the United States, Britain and France, which along with Russia and China, are veto-wielding members of the Council.

The key question is whether Russia, an ally of Syria, will support such a draft.

During Russian Deputy Foreign Minister Alexander Sultanov’s visit to Lebanon , he promised Russia will not veto the UN resolution on the tribunal , even if it did not agree with it. Sultanov who visited Syria after meeting with the Lebanese officials must have found out on his own how Syria’s objection to the tribunal , which is reflected through its allies in Lebanon.

In Beirut, US Middle East envoy David Welch voiced confidence Tuesday that the Security Council would impose the court.

Welch, the US assistant secretary of state for Near Eastern affairs is on the first visit to Lebanon by a US official since Israel's devastating war with Hezbollah last summer.

Asked whether imposing the tribunal on a divided Lebanon might not spark civil war, Khalilzad replied: "We understand that there are some risks but the risks of not taking action are greater."

In a letter addressed to the UN secretary general, Pro-Syrian Lebanese President Emile Lahoud meanwhile said Tuesday that "ratifying the international court by the UN Security Council would imply a full bypass of the constitutional mechanism in Lebanon."

He regretted that "the present (Lebanese) ruling party still insists on breaching the constitution and on begging the world's international organisation ... to meddle" in Lebanese internal affairs "and take sides with one party against the other."

And Lebanon's Damascus-backed Hezbollah also warned of "discord" if the UN imposes the tribunal.

The Siniora government accuses the pro-Damascus opposition -- led by Hezbollah -- of blocking parliament's ratification of the tribunal plan at the behest of Syria.

Damascus denies any links with the assassination. It has made clear it will not allow any Syrian to be tried by a court it regards as an affront to its sovereignty.

A UN enquiry panel, now headed by Serge Brammertz of Belgium, initially implicated senior Syrian officials and their Lebanese allies in the Hariri slaying.

Picture: US Ambassador to the UN Zalmay Khalilzad