UNITED NATIONS — As a politically explosive measure establishing an international tribunal to try suspects in Lebanon’s political assassinations came into effect yesterday, a senior U.N. official accused Syria of stoking violence in Palestinian Arab camps in northern Lebanon.

One of Secretary-General Ban’s top envoys to Lebanon, Terje Roed-Larsen, told the Security Council about reports from the Lebanese army and other sources that describe a “steady flow of weapons, provisions, and armed elements” across the Lebanese-Syrian border into the Palestinian Arab camp Nahr al-Bared.

He called the reports “alarming.”

Fighting erupted in Palestinian Arab camps last month, just as the council was considering the establishment of the tribunal that could try several high-ranking Syrian officials.

In addition to Fatah al-Islam — the Qaeda-affiliated militia that has fought from within the camp since May 20 — reinforcements from two other Palestinian Arab organizations joined the battled the Lebanese army, Mr. Roed-Larsen told the council.

The groups are the Popular Front for the Liberation of Palestine- General Command, and Fateh al-Intifada, he said. Both, he noted, are headquartered in Damascus.

According to Mr. Larsen, Lebanese army officials have reported that “one of the major buildups” of arms and fighters flowing across the Syrian border “occurred just before the resolution” in which the Security Council decided to establish the tribunal no later than yesterday — even if Lebanese politicians were unable to agree to its provisions by then.

“Mr. Larsen’s reports have always been partial,” Syria’s ambassador to the United Nations, Bashar Jaafari, told reporters. “He is not a welcomed official in Syria.”

Mr. Larsen has “provided false information to the council” many times, he added.

Yesterday’s report, which largely relied on information from Lebanon, should have been disseminated through officials in Beirut rather than by Mr. Larsen, Mr. Jaafari said.

The council intervened in the internal process of ratifying the international tribunal only after a months-long bid by pro-Syrian politicians to prevent the Parliament from voting on it. Arguing that legislators were “deadlocked,” Prime Minister Siniora asked for help from the council. A resolution last month provided one more chance for Lebanon to ratify the process, but that chance expired yesterday.

It would take about a year for the tribunal to become active, according to U.N. officials. International and Lebanese jurists need to be named, and while the Netherlands and Cyprus have been mentioned as possible hosts for the tribunal, neither country has agreed to this yet.

Also, the tribunal will be established only after sufficient funding is provided for the first year of its existence — which a U.N. official estimated yesterday at $30 million — with reasonable pledges for two additional years.

A U.N. diplomat who spoke on condition of anonymity yesterday said council members expected at least half of the tribunal’s costs to be funded by Arab countries.