The Israeli Northern Command, responsible for Israel's border with Lebanon and Syria, increased its state of readiness for military action against terrorism or military attack and told Israelis to stay away from the Lebanese border, Ynetnews reported.
The heightened alert state followed a warning from Hezbollah it would retaliate against Israel for carrying out an airstrike against one of the group's positions in Lebanon.
The attack was Israel's first against Hezbollah in eight years.
"On Monday night ... the Israeli enemy's warplanes bombarded a Hezbollah position on the Lebanese-Syrian border, near the area of Janta in the Bekaa Valley [in eastern Lebanon]," Hezbollah said in a statement on its website.
The statement was the militant group and political party's first acknowledgment Israel had struck a target inside Lebanon.
"This new attack is a blatant act of aggression against Lebanon and its land," Hezbollah said.
The movement, whose paramilitary wing is widely considered more powerful than the Lebanese army, said it would "not stand without a response" and would "choose the right time and place and the proper way to respond."
The Washington Post said the language in the warning suggested Hezbollah didn't plan to retaliate right away.
Israel did not say why it immediately heightened its alert state.
The Post said Israel's airstrike against Iranian-backed Hezbollah -- its first since the two sides fought a devastating but inconclusive 34-day war in 2006 -- underscored the danger the Syrian civil war could escalate into a region-wide conflict.
That conflict killed a reported 1,200 to 1,300 Lebanese, including combatants, and 165 Israelis, including soldiers.
Israel has not confirmed or denied Monday night's airstrike -- it has a longstanding policy of silence on pre-emptive military strikes.
But it has repeatedly vowed to prevent Hezbollah from acquiring sophisticated missiles.
Hezbollah denied the attack targeted a weapons convoy and denied there were casualties.
"This aggression did not, thank God, cause any deaths or injuries," the group said in its statement, which was also broadcast over Hezbollah satellite TV station al-Manar. "There was only some material damage."
Security sources told Lebanon's Daily Star Wednesday the strike targeted two trucks -- one carrying missiles and the other carrying a missile launcher.
The trucks were on their way to Hezbollah missile storage facility in Lebanon, the newspaper said.
U.S. Secretary of State John Kerry told MSNBC Washington was aware of "threats of weapons transfer from [Syrian President Bashar] Assad to Hezbollah and this is a threat to Israel."
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