SHEAR YASHUV, Israel — The Lebanese militant Hezbollah group fired a salvo of missiles at an Israeli military convoy in a disputed border area Wednesday, killing two soldiers and triggering deadly clashes that marked the most serious escalation since the sides’ 2006 war.

The flare-up, which left a U.N. peacekeeper dead and seven Israeli soldiers wounded, adds to the regional chaos brought on by neighboring Syria’s civil war. Hezbollah indicated the attack was in retaliation for a deadly Israeli strike on its fighters inside Syria earlier this month.

The violence sparked fears in both countries of yet another crippling war between the two foes. Israel’s Prime Minister Benjamin Netanyahu warned Israel would respond “forcefully,” and the military fired artillery shell barrages that struck border villages in southern Lebanon. Hezbollah responded with rocket fire on Israeli military positions in an exchange of fire that lasted for hours.

The Israeli military said five anti-tank missiles hit the soldiers as they were traveling near Mount Dov and Chebaa Farms, along a disputed tract of land where the borders of Israel, Lebanon and Syria meet.

The soldiers were in two unarmored white vehicles without military insignia when they were struck from a distance of about three miles away, according to Lt. Col. Peter Lerner, an Israeli military spokesman.

Israeli media aired footage showing the charred, smoldering vehicles after the strike.

The dead soldiers were identified as Capt. Yochai Kalangel, 25, and Sgt. Dor Chaim Nini, 20. Seven other soldiers were wounded, the military said.

In Washington, State Department spokeswoman Jen Psaki condemned the Hezbollah attack and expressed support for Israel’s “legitimate right to self-defense.”

Hezbollah said the operation was carried out by a group calling itself the “Righteous Martyrs of Quneitra,” suggesting it was to avenge an Israeli airstrike in the Syrian portion of the Golan Heights on Jan. 18 that killed six Hezbollah fighters, including the son of the group’s slain military commander, Imad Mughniyeh, and an Iranian general.

Israel has braced for a response to that strike, beefing up its air defenses and increasing surveillance along its northern frontier.

“Whoever stands behind today’s attack will pay the price in full,” a statement from Netanyahu’s office quoted him as saying. Netanyahu said that Iran, through Hezbollah, was working to establish a base in southern Syria from which to launch attacks against Israel. “We are working resolutely and responsibly against this attempt,” he said.

He said Syrian President Bashar Assad and the Lebanese government shared the blame for attacks against Israel emanating from their territory.