UNITED NATIONS (Reuters) - U.N. Secretary-General Antonio Guterres condemned civilian deaths in Gaza as "unacceptable" and appealed for them to "stop immediately" and for all parties to exercise maximum restraint, Deputy U.N. spokesperson Farhan Haq said on Wednesday.

Israel hit Islamic Jihad targets in Gaza for a second day on Wednesday and Palestinian militants launched hundreds of rockets across the border, setting off sirens as far away as Tel Aviv, while Egypt began efforts to mediate an end to the fighting.

The 15-member U.N. Security Council met behind closed-doors on Wednesday over the latest violence.

In total 20 Palestinians, including at least five women and five children, as well as three senior Islamic Jihad commanders and four gunmen have been killed since Israel launched pre-dawn strikes on Tuesday, Palestinian health officials said.

"Israel must abide by its obligations under international humanitarian law, including the proportional use of force and taking all feasible precautions to spare civilians and civilian objects in the conduct of military operations," Haq said.

Guterres also condemns the indiscriminate launching of rockets from Gaza into Israel, which violates international humanitarian law and puts at risk Palestinian and Israeli civilians, Haq said. 

"The Secretary-General urges all parties concerned to exercise maximum restraint and to work to stop hostilities immediately," Haq said in a statement.

(Reporting by Michelle Nichols; Editing by Rami Ayyub and Alistair Bell)