The United States accused Iran Tuesday of violating a U.N. arms embargo by secretly sending weapons to Syria in a cargo ship seized by Israel, a U.S. official said.

Britain took a softer line in the U.N. Security Council, expressing "very serious concern" at suggestions that Iran was caught illegally exporting weapons but saying it was waiting for more information about the ship's origin, destination, cargo and seizure.

U.S. deputy ambassador Alejandro Wolff and British deputy ambassador Philip Parham raised Israel's seizure of the cargo vessel Francop on Nov. 4 off Cyprus during a closed-door council debate on implementation of the 2006 cease-fire between Israel and Hezbollah militants in Lebanon that ended their 34-day war, council diplomats said.

Israel has not provided documentary evidence to back its claims that 36 containers of weapons hidden among hundreds of containers of civilian cargo on the Francop came from Iran and were headed for Lebanon's Hezbollah fighters via Syria. But its contention about the Iranian origin was bolstered by Iranian markings on the side of containers filled with rockets, missiles, mortars, anti-tank weapons and munitions shown to reporters in Israel.

The United States told the council that the concealed arms shipment, "clearly manifested from Iran to Syria" in violation of a March 2007 arms embargo, provides "unambiguous evidence of the destabilizing proliferation of arms in the region," the U.S. official said, speaking on condition of anonymity because the council meeting was private.

The United States also called on Syria and Iran to end their "material support" for Hezbollah and other militias in Lebanon, which violates the 2006 cease-fire resolution, the U.S. official said.

Syria's U.N. Ambassador Bashar Ja'afari countered by accusing Israel of committing "an act of piracy on the high seas," and saying it should be held legally accountable, with the issue followed by the Security Council.

"All these rumors circulated by the Israelis are ... the usual Israeli outrageous concoction of lies," he said.

Britain's Parham told reporters after the council meeting that the Francop appears to be the third case of illegal Iranian arms exports this year.

"We hope that Iran will play a part in achieving a secure, peaceful and prosperous Middle East, but its current behavior appears to point in a different direction," Parham said.

The council meeting on Lebanon took place three days after Lebanon's Syrian-backed factions finally agreed on a unity government proposed by their pro-Western rivals, ending a four-month deadlock in the deeply divided country.

Austria's U.N. Ambassador Thomas Mayr-Harting, the current council president, said members "welcomed the progress achieved by the formation of a new unity government."