JERUSALEM (CNN) -- An Israeli navy gunboat fired shells into northern Gaza on Friday, killing at least seven Palestinians on a beach, Palestinian medical sources said.

The Israel Defense Forces halted firing pending an investigation into the strike at Beit Lahya.

The IDF apologized and said it "regretted the strike on innocents," the Israeli newspaper Haaretz reported.

Haaretz said the IDF's chief of staff, Gen. Dan Harel, had ordered a halt in the shelling. The Israeli paper quoted Palestinian Authority President Mahmoud Abbas as condemning the killings as a "bloody massacre."

Separately, three Palestinian militants died earlier Friday in an Israeli airstrike, also in the Beit Lahya area, and two Palestinians were killed in another Israeli strike in the Jabalya refugee camp in Gaza.

Video footage from the beach showed ambulance workers carrying away injured people on stretchers. A young girl was screaming for her mother in the footage.

Palestinian security sources said about 20 others were injured when Israeli shells hit the beach.

The IDF said it was shelling rocket-launching areas in Gaza from the sea. A representative said the areas were believed to be uninhabited.

Earlier Friday at Beit Lahya, the IDF said militants launched a Qassam rocket that landed in Israel but caused no damage or casualties.

Members of the group got into a car, which the Israeli air force then attacked. A large explosion followed.

The IDF said it believed the blast was due to explosives in the car.

The three dead were members of the Popular Resistance Committees, a coalition of militant groups, Palestinian security sources said. Palestinian sources said the dead included two brothers and a cousin.