Islamist group Hamas said several of its top commanders were killed in Israeli strikes on Wednesday, including its military chief in Gaza City, Bassem Issa.

Israel's internal security agency, Shin Bet, named Issa as one of the four top Hamas militants who it said were killed.

Israeli Prime Minister Benjamin Netanyahu addressed the killing, noting he had warned that the lives of Palestinian militants in Gaza were at risk.

"This is just the beginning; we'll deliver them blows they haven't dreamt of," he said while visiting an Israeli girl wounded in a rocket attack at a hospital in the centre of the country.

According to Shin Bet, besides Issa the strike killed Jumaa Tahle, head of Hamas cyber command who was also in charge of improving rocket accuracy, head of research and munitions Jamal Zibde, and Hazem Hatib, the group's chief production engineer.

A dozen other members of the Hamas research and development division were also killed in a strike that destroyed their facility, Shin Bet said.

Gaza militants have launched more than 1,000 rockets since Monday, according to Israel's army, which has carried out hundreds of air strikes on Islamist groups in the crowded coastal enclave of Gaza.