Israel has put embassies on alert and stepped up security on its border with Lebanon after the killing of a top commander of militant group Hezbollah.
New security measures could remain in place for months
Hezbollah and Iran have blamed Israel for the death of Imad Mughniyeh, who died in bomb blast in Damascus on Tuesday night.
Israel has rejected the accusations while publicly welcoming news of Mughniyeh's death.
But officials said they were wary of the risk of reprisals.
"We have begun evaluating the situation with representatives of the army and police, with everyone who is in charge of security in cases like this," said Internal Security Minister Avi Dichter.
According to Israel's army radio, army chief Lieutenant General Gaby Ashkenazi had ordered ground, air and naval forces to be on alert.
Domestic security agency Shin Bet recommended heightened security for Israeli airline El Al, Israeli shipping, and synagogues around the world, the radio reported.
Security was also tightened at embassies, consulates and foreign offices of the Jewish Agency, officials said.
One official said the high alert could remain in place for weeks or even months.
Mughniyeh was linked to numerous attacks on Israeli and Western targets in the 1980s and 1990s.
Prime Minister Ehud Olmert's office issued a statement rejecting "the attempts of terror elements to attribute to Israel any involvement in this incident".