Israel carried out a series of airstrikes in the Nabatieh district, south Lebanon, in response to Hezbollah rocket fire near a watchtower, the first time the group has attacked Israel since a ceasefire came into effect Wednesday morning last week.

Hezbollah had launched two rockets near a watchtower in the occupied Shebaa farms earlier on Monday night. They landed in an open area and caused no injuries. In a statement, the group said the attack on the watchtowers was an “initial warning defensive response” against “repeated violations” by Israel of the ceasefire agreement.

An Israeli military spokesperson said that the army was attacking targets in Lebanon but would give “further details later.” An hour earlier, Israeli PM Benjamin Netanyahu vowed a “strong” response to Hezbollah’s attack.

The Israeli airstrikes constituted the most serious attacks since the ceasefire was established on Wednesday. The resumption of tit-for-tat strikes in south Lebanon and north Israel has caused concern that fully fledged fighting between Hezbollah and Israel could resume just five days after a ceasefire was announced.