The Israeli air force says it struck Hezbollah targets in southern Lebanon in response to cross-border rocket fire in the most significant flare-up of tensions since a ceasefire brought uneasy calm to the region.

The exchange sparked concern about whether the ceasefire would hold, days after Israel relaunched war with another Iran-backed militant group, Hamas, in Gaza.

Israeli artillery and airstrikes hit south Lebanon on Saturday after Israel said it had intercepted rockets fired from across the border, endangering a shaky truce that ended a year-long war between Israel and the Lebanese armed group Hezbollah.