The United States is pressing Palestinian leader Yasser Arafat for an explanation of what Israel says was its interception of a boat carrying some 50 tons of Iranian weapons bound for the Palestinian Authority.

U.S. envoy Anthony Zinni raised the issue at his meeting Friday with Mr. Arafat, who denied any knowledge of the shipment.

The report by Israel of the interception of the vessel in the Red Sea added a new complication to Mr. Zinni's latest effort to implement a U.S.-negotiated Israeli-Palestinian cease-fire agreement.

Briefing reporters, State Department spokesman Richard Boucher said the former U.S. Marine general asked Mr. Arafat to explain the incident and urged him to combat and dismantle what he termed the "terrorist infrastructure" in areas under the Palestinian Authority's control.

"In his meeting with Chairman Arafat today he expressed, as I said, our strong condemnation of any attempts to escalate the conflict in the region by militant groups or others," Mr. Boucher said. "[Mr. Zinni] pressed Chairman Arafat to find an explanation of the situation. He also urged Chairman Arafat to take immediate steps to prevent future attempts to bring in additional weaponry or escalate the current conflict."

Mr. Zinni reported that the Palestinian leader denied any knowledge of the shipment and offered full cooperation and an investigation.

Israel said the arms shipment, found aboard a ship being piloted by Palestinian Authority personnel, included long-range artillery rockets, anti-tank missiles, and other heavy weapons nominally forbidden to the Palestinians under the 1993 Oslo accords and follow-on agreements.

Asked here about the Iranian role in the shipment, spokesman Boucher said the facts in the case are not in. But he said Iran has long been listed by the United States as a supporter of militant and terrorist groups and said that "needs to be stopped."

He also said under questioning the United States was "not involved" in Israel's seizure of the ship, which occurred in international waters some 500 kilometers from the Israeli port of Eilat on the Gulf of Aqaba.

Mr. Zinni, who is on his second mission to the region since being named to the special envoy post in November, secured an agreement for the resumption of direct Israeli-Palestinian security talks.

He is due to return to Washington early next week to consult with Secretary of State Colin Powell on the next step in U.S. peace efforts.