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

Hezbollah has fired an estimated 13,000 rockets into Israel over the past year in support of Hamas’ war with Israel in Gaza. Tens of thousands of northern Israelis have been displaced from their homes, and Israel has said its escalating war with Hezbollah is aimed at stopping those rockets so families can return home.