More than 50 Hizbullah terror cells believed to be spread across the globe could be activated and used to strike at Israeli or Jewish targets in retaliation for Tuesday's assassination of Hizbullah arch-terrorist and operations officer Imad Mughniyeh in Syria, a senior defense official said Thursday.

As Hassan Nasrallah vowed "open war" to avenge Mughniyeh's death, the Israeli Counterterrorism Bureau issued a travel advisory on Thursday that made plain the seriousness with which it is taking the Hizbullah leader's threats. But Foreign Minister Tzipi Livni said "Israel is a strong country" and that there was no need to panic.

FBI anti-terror units raised their alerts for fear of attacks on synagogues and other Jewish targets.

Sources in the US administration reiterated that there were no specific warnings of a terrorist attack; nonetheless, an FBI source said that the raised alertness of the Anti-Terrorism Unit, which operates in about 100 cities around the US, is not a routine step.

The counterterrorism bureau recommended that citizens abroad avoid staying in areas where there is a large concentration of Israelis. It advised Israelis overseas to strictly avoid visiting Arab and Muslim states where existing travel warnings are in force; to reject any tempting suggestions, unexpected gifts and offers of free travel from suspicious people or unknown elements; to reject proposals for unscheduled meetings, and to travel to meetings accompanied by someone known and trusted.

The bureau also reiterated previous warnings about the risk of Israelis abroad being kidnapped, including businessmen - particularly those involved in deals with Arabs or Muslims. El Al has also beefed up security, and additional safety measures are being employed at Israeli embassies and institutions abroad.

At home, IDF Chief of General Staff Lt.-Gen. Gabi Ashkenazi ordered the Northern Command to raise the level of alert and beef up forces along the Lebanese border, for fear that Hizbullah might retaliate by kidnapping soldiers or attacking forces deployed along the border.

The defense establishment, The Jerusalem Post has learned, plans to hold daily security assessments over the coming days and the Shin Bet (Israel Security Agency) is considering deploying additional security personnel at embassies deemed at high risk.

Nasrallah delivered a videotaped eulogy at Mughniyeh's funeral in Beirut on Thursday and vowed to retaliate against Israeli targets abroad. Mughniyeh and Hizbullah were responsible for the deadly bombings of Jewish targets in Buenos Aires in the early 1990s.

"You have killed Hajj Imad outside the natural battlefield," Nasrallah said, in reference to Hizbullah's contention that it only fights Israel within Lebanon and along their common border. "You have crossed the borders," he continued. "With this murder, its timing, location and method... If you want this kind of open war, let the whole world listen: Let this war be open."

Nasrallah warned Israel that its alleged killing of Mughniyeh was a "very big folly" which would be avenged. "Mughniyeh's blood will lead to the elimination of Israel. These words are not an emotional reaction," he said.

Livni, speaking during a visit to Washington on Thursday, commented on Nasrallah's speech, saying "Israel is a strong country; the Jewish people are a strong people and our response to terror is clear. Comments from this or that terrorist will not change this and we will not enter into a state of panic.

"Hizbullah is a terrorist organization that has carried out terrorist attacks in the past and is trying to carry out attacks all the time. Israel has lived under threats from the day it was established. We know how to cope with such threats and will do so now," she asserted.