Israeli airstrikes have killed four Palestinians in the Gaza Strip and Palestinian militants fired more rockets into southern Israel in a fourth day of violence that drew concern from the international community.

Palestinian medics said Monday’s Israeli strikes on Gaza killed two militants and two civilians – a man in his sixties and his daughter.

The Israeli military said its aircraft hit several militant targets, including a weapons storage facility and rocket-launching sites.

Gaza militants fired at least 30 rockets and mortar shells at southern Israel during the day, with several landing inside Israeli communities. No casualties were reported. Israel says its Iron Dome missile defense shield intercepted seven of the rockets.

Israeli airstrikes on Gaza have killed 22 Palestinians, all but four of them militants, since the fighting erupted on Friday. Palestinian rocket attacks on southern Israel have wounded several Thai agricultural workers, forced schools to close and disrupted the lives of hundreds of thousands of Israelis.

U.N. Secretary-General Ban Ki-moon expressed grave concern about the latest escalation in violence between Gaza and Israel, saying civilians are paying a “terrible price.” In remarks to the U.N. Security Council on Monday, he said rocket fire on Israeli civilians is “unacceptable” and urged Israel to exercise “maximum restraint.”

U.S. Secretary of State Hillary Clinton also addressed the council, saying Washington condemns rocket attacks by Gaza “terrorists” in the strongest terms and calls on those responsible to take immediate action to stop them.

In a separate statement, the Quartet of Mideast peace mediators that includes the United Nations, the United States, the European Union and Russia appealed for calm in the region after meeting at the U.N. headquarters in New York.

An Israeli military spokesman said the airstrikes will stop if militants end their rocket fire. But, he also said Israeli forces will continue to take pre-emptive action against militants trying to stage further attacks on Israel.

Israel’s killing on Friday of a Palestinian militant leader whom it accused of plotting an attack touched off the latest round of violence.

Palestinian militant group Islamic Jihad has claimed responsibility for many of the more than 100 rockets fired at Israel in the latest fighting. In a statement Monday, the group vowed to continue firing rockets at Israeli cities as long as Israel refuses to stop targeting militants.

Islamic Jihad also criticized efforts by some Palestinians to secure a truce with Israel through Egyptian mediators, saying those people should exert pressure on the Israeli “enemy” rather than Gaza’s “resistance” fighters.

A spokesman for the Hamas militant group that runs Gaza told the Reuters news agency that he expects the situation to calm down soon. Mahmoud Al-Zahar said Hamas does not want an escalation in fighting.