* Syrian-made arms would have boosted Palestinian firepower
* Announcement comes as Netanyahu makes U.S. lobbying visit (Adds Iranian denial)
By Dan Williams
JERUSALEM, March 5 (Reuters) - The Israeli navy seized a ship in the Red Sea on Wednesday that was carrying dozens of advanced Iranian-supplied rockets made in Syria and intended for Palestinian guerrillas in the Gaza Strip, the military said.
The announcement came as Israeli Prime Minister Benjamin Netanyahu was in the United States to press his case for tougher international action against Iran over its disputed nuclear programme and support for Islamist guerrilla groups.
The Panamanian-flagged cargo vessel Klos C was boarded in international waters without resistance from its 17-strong crew in a "complex, covert operation", military spokesman Lieutenant-Colonel Peter Lerner told reporters.
Lerner said dozens of M302 rockets were found aboard the Klos C, a weapon that could have struck deep into Israel from Gaza and would have significantly enhanced the firepower of the Palestinian enclave's Hamas rulers and other armed factions.
"The M302 in its most advanced model can strike over 100 miles, and if they would have reached Gaza, ultimately that would have meant millions of Israelis under threat," he said.
Hamas dismissed the Israeli announcement as a "silly joke".
"This is a new Israeli lie aimed to justify and prolong the blockade of Gaza," said Taher Al-Nono, adviser to the Hamas prime minister, Ismail Haniyeh.
There was no immediate comment from Syria, but Iran denied the accusations.
"This allegation is not true and in principle the message or movement of a ship carrying weapons from Iran to Gaza is not true," said Amir Abdollahian, Deputy Foreign Ministry for Arab and African Affairs, according to the state news agency IRNA.
"The allegation is merely based on repetitious and baseless fabrications of the Zionist media."
U.S. State Department spokeswoman Jen Psaki said the United States had helped Israel by providing information on the ship.
"Soon after becoming aware of the imminent movement of the suspected vessel, the White House directed the Department of Defense to monitor the vessel," she said.
Psaki said U.S. options for handling the ship had included taking unilateral action if necessary but that, after sharing intelligence, Israel chose to take the lead in the operation.
Military footage showed the Israeli navy chief, Admiral Ram Rothberg, inspecting a rocket on the floor of a ship's hold, with cement bags labeled "Made in Iran" in English next to it.
Lerner said the rockets had been flown from Syria to Iran, from where they were shipped first to Iraq and then towards Sudan. Had they reached the African coast, they would probably have been smuggled overland through Egypt to Gaza, he said.
Nic Jenzen-Jones, an Australia-based military arms specialist and director of Armament Research Services, said most reports indicated the Syrian-produced rockets had a 90- to 100-km (55- to 60-mile) range.
"Several Israeli assessments of these rockets have questioned their reliability," he said. "(The Lebanese Shi'ite group) Hezbollah has made use of these rockets, and Hamas is believed to be attempting to stockpile longer-range rocket systems."
Israel and Hamas last fought a major conflict in November 2012. Hamas has largely held fire since, but Israel says it has been trying to build up its capabilities. That has been made difficult, however, by a new military-backed government in Cairo, which has toughened controls on Egypt's border with Gaza.
Netanyahu's office said the prime minister, who was in Los Angeles on Wednesday after holding a meeting in the White House and addressing a pro-Israel lobby in Washington, had approved the seizure after consultations with his security chiefs.
"At the same time that it is talking to world powers, at the same time that Iran is smiling and saying all kinds of honeyed words, that same Iran is sending lethal weaponry to terrorist organisations and it is doing so in a complex web of covert, worldwide operations," Netanyahu said from Los Angeles.
Foreign Minister Avigdor Lieberman said Israel had obtained Panama's permission to board the ship.
"We followed international law to the letter. The ship travelled under a Panamanian flag, the company was listed in Marshall Islands, the captain was Turkish and the crew was from various different countries," he told a conference in Tel Aviv.
In a speech to the AIPAC lobby on Tuesday, Netanyahu had reiterated his unhappiness with the prospect that world powers negotiating a curb on Iran's nuclear programme would let it retain some technologies with bomb-making potential.
Iran denies seeking nuclear weapons and accuses its arch-foe Israel, widely believed to be the only nuclear-armed country in the Middle East, of being the real regional menace.
Lerner said Iran had orchestrated the shipment, and that the Klos C was being brought to the Israeli port of Eilat, where its contents would be more fully inspected and displayed to the public.
Lerner said there was no immediate indication that the crew had known the nature of their cargo.
According to tracking data, the Klos C docked at Bandar Abbas in Iran in early February, and at Port Said in Egypt in January. (Additional reporting by Jonathan Saul in London and Nidal al-Mughrabi in Jerusalem and Bill Trott in Washington; Editing by Crispian Balmer, Ralph Boulton, David Gregorio and Kevin Liffey)