UNITED NATIONS — In one of the first tests of his diplomatic skills as America’s new ambassador here, Zalmay Khalilzad is about to face a Security Council showdown on Lebanon that could have wide implications for the Middle East and the war on terror.

Mr. Khalilzad — an alumnus of the American University in Beirut — assumed the monthly rotating presidency of the council May 1, on only his second week as ambassador, and one of his most challenging tasks, as emerged yesterday, is to unite members on the issue of Lebanon.

At issue is an attempt to pass a new resolution that would enforce the council’s past decision to set up an international tribunal to try suspects in the 2005 killing of a former prime minister, Rafik Hariri, and other political assassinations that have taken place in Lebanon since.

“The process in Lebanon to establish the tribunal is deadlocked, and the council needs to look at options to assist the Lebanese, because it’s very important that this tribunal will be established as soon as possible,” Mr. Khalilzad told reporters. “It’s important for reasons of justice and it’s important for deterrence of such actions in the future, and therefore important for the long term stability of Lebanon.”

The tribunal, which was set up by the council last year, is widely supported in Lebanon. America and France see it as a necessary tool in liberating the country from Syria’s yoke. But several members of President Assad’s family and his government may end up as suspects, and Damascus has marshaled its political allies in Lebanon to resist the establishment of the tribunal.

These pro-Syrian politicians have blocked a parliamentary vote to ratify the tribunal, which has been approved by the government of Prime Minister Siniora. Mr. Siniora and, separately, a majority of parliament members, have sent letters to the council recently, asking that it intervene to break the impasse. But Syria’s ally, Russia, has resisted a council move to enforce the establishment of the tribunal.

Mr. Khalilzad spoke yesterday after a U.N. legal adviser, Nicolas Michel, reported to the council that he has failed in his latest attempts to assist in breaking the Lebanese political impasse. Following several trips to the country, in which he met with politicians of all parties, Mr. Michel told reporters yesterday, “From all the discussions that I have, all the efforts that I attempted, I see no progress.”

According to several diplomats, Mr. Michel’s trips — as well as an unsuccessful Damascus visit by Secretary General Ban last week — were designed to demonstrate to Syria’s allies on the council, led by Russia, that the impasse is intractable and that the only way to set up the tribunal is to pass a new resolution under Chapter Seven of the U.N. Charter, which allows enforcement of the council’s will.

America, France and Britain, as well as most other of the 15 members, want the council to move to a Chapter Seven resolution now, so the tribunal can quickly be established outside Lebanon, most likely in Cyprus. Although Russia has stopped short of threatening a veto, it has resisted attempts to pass such a resolution.

“People in Lebanon cannot be looking to the security council to solve all their problems,” Russia’s U.N. ambassador, Vitaly Churkin, told reporters yesterday. “We do not share the enthusiasm for a Chapter Seven resolution,” added another Russian diplomat, who asked not to be named.

“If there is no place to put them on trial, why did we conduct the investigation in the first place?” said a European diplomat who asked for anonymity. The Russians “are being difficult,” the diplomat added.