America's top general made an unannounced visit to Israel on Friday for talks on ‘Iran and regional security challenges’, Israel's army said as it confirmed a reinforced presence on its northern border.

General Mark Milley, the chairman of the Joint Chiefs of Staff, held talks with Israeli Prime Minister Benjamin Netanyahu, alternate premier and Defence Minister Benny Gantz and Israel's army chief Aviv Kohavi.

Earlier on Friday, explosions on the Syrian side of the security fence in the Israeli-occupied Golan Heights caused damage to a vehicle and a civilian building on the Israeli side, an army statement said.

The army said it was investigating the incident, and it was not immediately clear if there had been an attempted attack on Israeli positions from within Syria. 

On Monday, five Iran-backed fighters were killed in an Israeli missile strike south of the Syrian capital Damascus, according to the Syrian Observatory for Human Rights.

The Iran-backed Lebanese Shiite militant group Hezbollah, an ally of the Syrian regime, said one of its fighters was among the dead.

Israel has launched hundreds of strikes in Syria since the start of the country's civil war in 2011, but rarely comments on such operations.

The Israeli army then announced an initial reinforcement of the northern border on Thursday, with Israeli media reporting the move was made in response to threats of retaliation from Hezbollah.

A further troop reinforcement on the northern border was announced on Friday, with the army saying it had "elevated its readiness against various potential enemy actions".

"The Israel Defence Forces holds the Lebanese Government responsible for all actions emanating from Lebanon," the statement said, without making reference to Hezbollah.