Image copyright AFP

Israel says it has fired on Hezbollah militants who crept into its territory, in what Israel's prime minister called a "serious security incident".

The Israel Defense Forces (IDF) said up to four militants crossed the border in the Mount Dov area, part of the Israeli-occupied Syrian Golan Heights.

The IDF said "the terrorists fled back to Lebanon" after being shot at.

The area has been tense for days after a Hezbollah fighter was killed in an alleged Israeli air strike in Syria.

Israel, which has neither confirmed nor denied it carried out the strike in the early hours of last Monday, had warned Hezbollah not to retaliate.

Israeli media, citing unnamed military sources, said the Hezbollah cell was planning to attack an IDF post. The reports said Israel had been tracking them and its forces opened fire once the militants crossed the so-called Blue Line - the UN-recognised boundary between Israel and Lebanon.

Reports say the Israeli army fired artillery shells in their wake. There are no reports of casualties on the Lebanese side.

Shortly before the incident, Israeli Prime Minister Benjamin Netanyahu warned that Israel would hold both Hezbollah and Lebanon responsible for any attacks from Lebanese territory.

"The IDF is prepared for any scenario," he said. "We are active in all arenas for the security of Israel - both close to our borders and far from them."

Hezbollah, which is heavily armed and funded by Israel's arch-foe Iran, is the most powerful armed force in Lebanon alongside the Lebanese army. It operates predominantly in the south of the country and along with its political allies is an influential force in the government.

Israel and Hezbollah are bitter enemies who fought a month-long war in 2006 after Hezbollah killed eight Israeli soldiers and abducted two in a cross-border raid.

The ensuing conflict killed some 1,191 people - mostly civilians - in Lebanon, and 121 soldiers and 44 civilians in Israel.