Lebanese Shiite clerics listen to Hezbollah leader Hassan Nasrallah, who makes speech to crowd through giant screen, during funeral of slain top commander Imad Mughniyeh, 14 Feb 2008

The leader of the Shi'ite militant group Hezbollah has threatened "open war" on Israel and accused the Jewish state of assassinating a top Hezbollah commander.

Hassan Nasrallah made his remarks in a fiery speech broadcast Thursday to thousands of Hezbollah supporters at Imad Mughniyeh's funeral in Lebanon's capital, Beirut.

He said Tuesday's car bomb assassination of Mughniyeh in Syria's capital, Damascus, occurred outside what he called the natural battlefield with Israel - implicitly the Israel-Lebanon border region. He said if Israel wants "open war" with Hezbollah, it will have it.

Israel has denied involvement in the car bombing.

Mughniyeh - one of the world's most wanted terrorists - was implicated in deadly attacks against Western and Israeli targets in the 1980s and 90s.

The U.S. State Department said Nasrallah's threats should be alarming to everyone.

State Department spokesman Sean McCormack said Hezbollah has a long record of carrying out terrorist acts around the globe. He said any time the militant group makes threats against Israel, the whole world should be concerned.

Israel put its embassies and other interests around the world on high alert today, fearing revenge attacks. Israel also reinforced its military forces along the Lebanese border.

Iranian Foreign Minister Manouchehr Mottaki attended the funeral for Imad Mughniyeh.

Sean McCormack said Mottaki's presence at the funeral underscores "the historical links between Iran and its terrorist group creation Hezbollah." Iran is Hezbollah's top ally.

Some information for this report was provided by AFP, AP and Reuters.