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Speaking to Israel's Channel 13 news, the analysts added no further details due to heavy censorship around the tense security situation.

"Our assessment is that the United States informed Israel about this operation in Iraq, apparently a few days ago," Barak Ravid, a journalist closely linked to Israeli security establishment told Channel 13 on Friday evening.

An Israeli army officer speaking on condition of anonymity also told the Los Angeles Times that the operation to take out Soleimani "did not come as a surprise".

Comment: Soleimani assassination spells trouble for Iraqis, Iranians and the region

Israel has been on high alert since Friday's strike, holding emergency security talks later the same day and Prime Minister Benjamin Netanyahu cutting short a trip to Greece.

Defence Minister Naftali Bennett chaired a meeting of security chiefs, including the heads of the army, the National Security Council and the Mossad intelligence agency, his office said.

Netanyahu broke off an official visit to Athens and flew home, expressing support for the overnight US strike that killed General Qasem Soleimani in Baghdad. 

"Just as Israel has the right of self-defence, the United States has exactly the same right," Netanyahu said as he boarded his flight from Athens.

"President (Donald) Trump deserves all the credit for acting swiftly, forcefully and decisively," he said. "Israel stands with the United States in its just struggle for peace, security and self-defence."

On the ground, the Israeli army closed Mount Hermon ski resort on the annexed Golan Heights, a disputed territory which borders Syria and Lebanon.

Fighters of the Iran-backed Lebanese Shia group Hezbollah, Israel's bitter foe with which it fought a devastating war in 2006, are deployed on the other side of the armistice line from which Israel fears reprisal attacks.

Although an Israeli military source said there were no new troop deployments, tanks and soldiers sealed off access to the Hermon site, while a battery of the Iron Dome missile defence system was also spotted.

Comment: A perilous turning point in the US-Iran confrontation

Late on Friday, however, the army announced the site would be opened as usual on Saturday, "in accordance with a situation assessment".

The heightened state of alert came after Iran's supreme leader Ayatollah Ali Khamenei vowed "severe revenge" for Soleimani's killing, the biggest escalation yet in a feared proxy war between Iran and the US on Iraqi soil. 

He was echoed by the leader of Hezbollah.

"Meting out the appropriate punishment to these criminal assassins... will be the responsibility and task of all resistance fighters worldwide," Hassan Nasrallah said.

Soleimani, head of the Quds Force in Iran's elite Revolutionary Guards, had long also been in Israel's sights for his alleged links to attacks on Israeli and Jewish targets worldwide.

Among them, Israel's Haaretz daily said, were the 1994 bombing of a Jewish community centre in Buenos Aires and an attack on an Israeli tour bus in Burgas, Bulgaria in 2012.

Agencies contributed to this report.