UNITED NATIONS — As Beirut verged on civil war in the aftermath of the murder of the Christian politician Pierre Gemayel, the Security Council yesterday set up a new international tribunal to try suspects in Lebanon’s political assassinations. The council’s decision was designed to strengthen Gemayel’s fellow pro-democracy forces in the country. Pending Lebanese approval, the decision is designed to strengthen pro-democracy forces.

Several Lebanese politicians and Gemayel relatives said Syria was behind yesterday’s assassination. Top American officials pointed out as well that ever since the 2005 killing of a former prime minister, Rafik Hariri, assassins in Lebanon have targeted outspoken critics of Syria and of Hezbollah, which is an ally of Syria and Iran.

America supports the efforts of the Lebanese people to “defend their democracy against attempts by Syria, Iran, and allies to foment instability and violence in that important country,” President Bush said in Honolulu. In Damascus, however, the Syrian government condemned and denied any involvement in the assassination.

Gemayel was gunned down point blank while driving his gray Kia in the Christian neighborhood of Jdeideh in northern Beirut. After a Honda CRV rammed into Gemayel’s car, at least three gunmen leaped out and sprayed it with bullets from silenced automatic weapons, the Daily Telegraph reported. Gemayel’s driver was also killed.

The minister of industry in Prime Minister Siniora’s government, Gemayel was at 34 one of the rising stars among a new generation of politicians of the main Christian Lebanese faction, the Phalange party. A scion of a Christian clan that has fielded some of Lebanon’s most prominent politicians — including his uncle Bashir, who served as president when he was slain in 1982 — Gemayel was a frequent critic of Syria and Hezbollah.

Gemayel’s father, Amin, himself a senior Phalange politician, called for calm in the aftermath of yesterday’s assassination. But other critics of Syria vowed to confront the assassins.

“We believe the hand of Syria is all over the place,” said Saad Hariri, a Sunni politician and the slain prime minister’s son. On CNN, Mr. Hariri characterized Gemayel as “a friend, a brother to all of us,” and promised, “we will bring justice to all those who killed him.”

The recent Lebanese power crisis has pitted Christian and Shiites supporters of Syria and Iran against pro-independence Sunni and Christian forces. The two sides differ on whether to accept or reject a plan, set up by Secretary General Annan, to establish an international tribunal. The tribunal is expected to try suspects in the Hariri and other political assassinations.

Bashar Al-Assad’s government in Damascus was recently seen by some as advancing toward rehabilitation. Syria announced yesterday the renewal of diplomatic relations with Iraq. There were also calls in Washington and European capitals to include gestures toward Damascus in regional diplomacy.

Several supporters of Syria and Hezbollah said yesterday that these trends make it unlikely that Damascus was behind Gemayel’s murder. The same argument was used, however, to exonerate Syrian officials in the Hariri case.

Nevertheless, at least one U.N. investigator, Detlev Mehlis, has cited leads that linked Mr. Assad’s brother in law, Assef Shawkat, and his brother Maher to the assassination. And according to a subsequent U.N. report, prepared by Mr. Mehlis’s successor Serge Brammertz, almost a dozen other assassinations fit the pattern of the Hariri killing.

“If you look at the reports of Mehlis and Brammertz over the months, the evidence that links the Hariri assassination to the other political assassinations, I think people can draw their own conclusions,” the American U.N. ambassador, John Bolton, told reporters yesterday.

“The Americans are used always to fish in the murky waters of the Middle East, instead of seeking with other influential actors a just solution,” countered the Syrian U.N. ambassador, Bashar Jaafari. Mr. Bolton’s “allegations have a political character,” he told The New York Sun. “They have no legal dimension.”

The Security Council’s endorsement yesterday of Mr. Annan’s tribunal blueprint strengthened the independence-minded forces led by Prime Minister Siniora, who vowed yesterday to try Hariri’s assassins, despite what he described as attempts at intimidation.

“This attack against a symbol of freedom in Lebanon,” Mr. Siniora said in a speech, referring to Gemayel, “makes us more determined to set up the international court, the tribunal that would stop the criminals and is the means to protect all Lebanese.”

But the plan is opposed by Syrian allies in Lebanon, led by the president, Emile Lahoud, a Christian. Five Shiite ministers, including all the Hezbollah members, resigned recently from Mr. Siniora’s government under Syrian pressure to oppose the international tribunal.

Echoes of this Lebanese debate were heard at the Security Council yesterday, as Russia and the Arab representative at the council, Qatar, fought to include a reference to the Lebanese constitution in a letter sent by the 15-member body to Mr. Annan, endorsing his tribunal plan. Some in Lebanon and on the council argue that according to the constitution Mr. Lahoud’s endorsement is needed for the tribunal to be legal.

Syria forced a change to the constitution in 2004, which has allowed President Lahoud to flout term limits and remain in office. That Syrian interference in Beirut’s politics prompted the Security Council to pass resolution 1559. Shortly afterward, after the Hariri assassination, Lebanon was set on the independence path known as the Cedar Revolution.