JERUSALEM, June 3 (Xinhua) -- Israel struck Hamas sites in the Gaza Strip on Sunday before dawn after Gazan militants fired rockets at Israel, the Israeli army said, breaking a three-day de facto ceasefire.

An Israeli military spokesperson said that some four projectiles were fired at southern Israel overnight, with three of them intercepted by Israel's Iron Dome anti-rocket system.

The spokesperson said that in retaliation, Israeli fighter jets hit 10 sites in three military compounds belonging to Hamas, the Islamist Palestinian organization that runs the besieged enclave.

Later, Israeli aircraft carried out additional airstrikes in southern Gaza, striking five targets at a compound belonging to Hamas' naval force, the spokesperson said.

The Israeli military said in a statement that the "IDF (Israel Defense Forces) views the Hamas terror organization's actions with great severity and will continue fulfilling its mission to protect Israeli civilians as necessary."

There were no immediate reports of casualties on both sides.

Israel and militant organizations in Gaza reached a de facto ceasefire last week, after a flare-up during which dozens of mortars and rockets were fired at southern Israel and Israel pounded Gaza with dozens of airstrikes.

The violence was part of a serious escalation in the border area between Gaza and Israel over the past months. Palestinian protests, which started as the Great March of Return on March 30 demanding Palestinian refugees' right to return home, escalated after the relocation of the U.S. embassy from Tel Aviv to Jerusalem. At least 121 Palestinians in Gaza have been killed in clashes with Israeli forces.?