An Israeli Apache helicopter firing a missile. REUTERS/Baz Ratner

An Israeli helicopter strike in Syria killed a commander from Lebanon's Hezbollah and the son of the group's late military leader Imad Moughniyah, sources close to Hezbollah said, in a major blow that could lead to reprisal attacks.

Iran confirmed that one of their generals died in the airstrike.

The strike hit a convoy carrying Jihad Moughniyah and other Hezbollah members including commander Abu Issa, in the Syrian province of Quneitra, near the Israeli-occupied Golan Heights, Lebanese sources said, killing five Hezbollah members in all.

It comes just days after Hezbollah leader Sayyed Hassan Nasrallah said frequent Israeli strikes in Syria were a major aggression, that the group was stronger than before and that Syria and its allies had the right to respond.

Shi'ite Muslim Hezbollah, which fought a 34-day war with Israel in 2006, has been fighting alongside President Bashar al-Assad's forces in Syria's four-year civil war.

"Hezbollah guerrillas in towns and villages along the border with Israel went on high alert, said an official from the group," The New York Times reports. "In the Shiite-dominated areas of south Lebanon and Beirut the streets emptied quickly, as residents feared an escalation."

The Hezbollah-run al-Manar news channel said the attack suggested "the enemy has gone crazy because of Hezbollah's growing capabilities and it could lead to a costly adventure that will put the Middle East at stake".

Senior Israeli military official says if Hezbollah responds to todays strikes with rockets, there will be a "very heavy response." — Israel Breaking (@IsraelBreaking) January 18, 2015

Hezbollah (and Iran) need to decide if Israeli strike enough of a reason to plunge all of Lebanon into a severely crippling war. — Daniel Nisman (@DannyNis) January 18, 2015

Israel's military declined to comment, but an Israeli security source confirmed to Reuters that the Israeli military had carried out the attack.

It was not immediately clear what role Jihad Moughniyah, in his 20s, was playing in the fighting in Syria.

Hezbollah had accused Israel in 2008 of assassinating his father Imad Moughniyah, who was head of the Hezbollah security network and accused of involvement in high-profile attacks on Western targets. Israel denies any involvement in that killing.

Nabil Boumonsef, a columnist at the Lebanon newspaper an-Nahar, said he believed the strike was a direct response to Nasrallah's speech and could lead to a backlash.

"Killing the son of Moughniyah is dangerous. I do not think that the group can be quiet now, now that the father and the son are killed. I expect that it will do something,” he said.

U.N. peacekeepers intensified their patrols on the border between Lebanon and Israel on Sunday night, local sources said.

Lebanon's Hezbollah members carry the coffins of two of their comrades during their funeral in Baalbek, in the Bekaa valley October 6, 2014. Ahmad Shalha/Reuters

RETALIATION THREAT

Al-Manar did not mention Moughniyah or the commander but confirmed that a number of fighters were killed when they were checking an area in Quneitra when there convoy came under Israeli missile attack. It said Hezbollah would announce the names of the dead later.

The area where the strike occurred. Google Maps Quneitra has seen heavy fighting between forces loyal to Assad and rebels including fighters linked to al Qaeda. Syrian state television said six people were killed in the attack and a child was wounded, without giving further details.

Israel has struck Syria several times since the start of the war, mostly destroying weaponry such as missiles that Israeli officials said were destined for Hezbollah, Israel's long-time foe in neighbouring Lebanon.

Syria said last month that Israeli jets had bombed areas near Damascus international airport and in the town of Dimas, near the border with Lebanon.

Imad Moughniyah, who was on the United States' most wanted list for the attacks on Israeli and Western targets, was killed in a car bombing in Damascus in 2008.

He was implicated in the 1983 bombings of the U.S. Embassy and U.S. Marine and French peacekeeping barracks in Beirut, which killed over 350 people, as well as the 1992 bombing of the Israeli embassy in Buenos Aires and the kidnapping of Westerners in Lebanon in the 1980s.

The United States indicted him for his role in planning and participating in the June 14, 1985, hijacking of a U.S. TWA airliner and the killing of an American passenger.

Nasrallah said on Thursday "the frequent attacks on different sites in Syria is a major breach. We consider (those) hostilities (to be) against all the resistance axis."

"(Retaliation) is an open issue ... When this right will be executed is subject to certain criteria ... it could happen any time."

Iran, Syria, Hezbollah and some Palestinian factions consider themselves an "axis of resistance" against Israel.

(Reuters reporting by Mariam Karouny, Laila Bassam, Oliver Holmes in Beirut, Dan Williams and Allyn Fisher-Ilan in Jerusalem; Editing by Kevin Liffey)