The Israeli navy intercepted a ship carrying weapons 100 nautical miles (185 kilometres) off its coastline overnight, a military spokeswoman said on Wednesday.
"During the night a special marine force intercepted a ship that was supposed to be carrying cargo around 100 (nautical) miles from our shore," the spokeswoman said, adding that the vessel was sailing under an Antigua flag.
"We suspected it was carrying weapons and when we inspected it that turned out to be true," she said, adding that the ship had been taken to port for further investigation.
The spokeswoman declined to say what kind of weapons were on board or where the ship was heading.
Israel has long accused arch-foes Syria and Iran of supplying weapons to Palestinian militants in the Gaza Strip, which has been ruled by the Islamist Hamas movement since June 2007.
On Tuesday a senior Israeli general warned that Hamas had successfully test-fired a rocket capable of reaching Tel Aviv from the Gaza Strip, potentially putting millions of Israelis within range of the projectiles.
The rocket, believed to be Iranian-made, has a range of about 60 kilometres (37 miles), putting Israel's major population centres in range, said Major General Amos Yadlin, head of military intelligence.
Gaza's Hamas rulers apparently successfully test-fired the rocket out to sea, Yadlin told a closed-door meeting of parliament's foreign affairs and defence committee, according to a participant.
Hamas called the claim a "fabrication" designed to mobilise world opinion against the Islamist group before the UN General Assembly which was on Wednesday to discuss a controversial report on the Gaza war.
"This is a pre-emptive step by the Zionist enemy to influence international opinion," Hamas spokesman Fawzi Barhum said, adding that the report had put Israel in a state of "crisis."
The UN report by respected South African jurist and former international war crimes prosecutor Richard Goldstone accused both Israel and Palestinian militants of committing war crimes during the December-January Gaza war.
Some 1,400 Palestinians and 13 Israelis were killed in the three-week war launched by Israel on December 27 and aimed at halting rocket attacks.
Militants have in recent years fired thousands of rockets into southern Israel, most with a range of just a few kilometres.
Yadlin said that although Hamas is trying to expand its arsenal, it does not appear ready for another conflict with Israel following last winter's war, but is instead focused on consolidating its rule over Gaza.
Israel has in the past seized shipments of weapons allegedly bound for Gaza, including in May 2003, when it intercepted a ship off its northern coast loaded with bomb-making material from the Lebanese Hezbollah militia.
On January 3, 2002, Israel intercepted a 50-tonne shipment of Iranian weapons aboard the Karine A, destined for the Palestinians in the Red Sea.
The late Palestinian leader Yasser Arafat admitted responsibility for the smuggling attempt, and the affair seriously eroded his standing with Washington.
On May 7, 2001, the navy intercepted the Santorini, which was packed with 40 tonnes of anti-aircraft missiles, Katyusha rockets, anti-tank grenades, mortar shells and automatic weapons, all bound for Palestinians in the Gaza Strip.
