Israel expanded its targets in its war with Hezbollah in Lebanon, killing at least 21 people in an air attack in the north, health officials said, while millions of Israelis took shelter from projectiles fired back across the border.

So far the main focus of Israel’s military offensive in Lebanon has been in the south, the Bekaa Valley in the east and the suburbs of the Lebanese capital, Beirut.

The strike in the Christian-majority town of Aitou on Monday hit a house that had been rented to displaced families, said Mayor Joseph Trad. At least eight people were injured, according to the Ministry of Public Health.

Rescue workers at the site of the attack searched through piles of rubble, where burned vehicles and trees could be seen strewn across the ground.

It was the first time the Christian-majority area had been attacked by Israel in a year of hostilities.

Israel ordered residents of 25 villages in southern Lebanon to evacuate to areas north of the Awali River, which flows some 60km (35 miles) north of the Israeli frontier.

Prime Minister Benjamin Netanyahu, visiting a military base in central Israel where four soldiers were killed on Sunday by a Hezbollah drone strike, said Israel would continue to attack the Iran-aligned movement “without mercy, everywhere in Lebanon – including Beirut”.

Israeli attacks have killed at least 2,309 people in Lebanon over the last year, the Lebanese government said in its daily update. The majority have been killed since late September when Israel expanded its assault, in which its forces have also attacked United Nations peacekeepers.