The Iran-backed Lebanese militant group Hezbollah has attacked an Israeli army base and a nearby military vehicle as tensions remain high along the border between Israel and Lebanon.

Israel says there were no casualties in the attack, and says it responded with artillery strikes on nearby Hezbollah positions, in the most serious single exchange across the border in more than four years.

The Israel Defense Forces (IDF) have been on high alert in the north of the country for the past week after Hezbollah leader Hassan Nasrallah vowed revenge for several attacks on Hezbollah targets in Lebanon and Syria last weekend.

Shortly after 4 pm local time on Sunday, the IDF says a Hezbollah squad fired anti-tank missiles at an Israeli battalion headquarters near the village of Avivim, hitting a building and a military ambulance but causing no casualties.

The IDF says it responded with around 100 artillery shells and “very limited” helicopter fire, targeting the area from where the missiles were fired, as well as other military sites.

It is the most serious cross-border exchange since January 2015 when Hezbollah killed two Israeli soldiers travelling in a jeep with an anti-tank missile, days after Israel had killed an Iranian general, the son of a senior Hezbollah commander and several others in an airstrike on a convoy of cars in Syria.

Prime Minister Benjamin Netanyahu, who also serves as the country’s defense minister, said Israel was now watching to see what Hezbollah does next. “We will decide on the next steps pending developments,” he said in a statement.

The IDF remains on elevated alert in the border area, but as of 8 pm on Sunday evening there were no restrictions on movements of Israeli civilians in the area.

The IDF has added to the sense that the immediate exchange of fire might be over by saying, “the tactical event on the ground appears to be behind us,” but adding, “however the strategic situation is still ongoing.”

Hezbollah says it carried out attack

Hezbollah acknowledged it had carried out Sunday’s attack; a statement from the Lebanese militant group say it had struck at 4:15 pm local time.

In a precursor to the exchange, and indicative of the heightened tensions in the border area, the Lebanese army reported earlier on Sunday that an Israeli drone had breached Lebanon’s airspace and dropped incendiary bombs, starting a fire, Lebanon’s official news agency reported.

The IDF confirmed the fires were “a result of IDF operations in the area.”

Israel acknowledged carrying out an airstrike on a building near Damascus last Saturday which killed two Hezbollah fighters, saying the operation was necessary to thwart a drone attack commanded by Iran.

Hezbollah also accused of Israel of responsibility for a drone operation of its own in Beirut last Sunday, though the IDF has not commented on that incident.

Three days ago, the IDF unveiled a detailed assessment of what it says are Iran’s attempts to assist Hezbollah upgrade its rockets into precision-guided missiles, through the transfer of advanced technology into Lebanon.

Lebanon meanwhile has called on France and the United States to intervene in the light of the escalation of tensions with Israel.