Lt. Col. Peter Lerner, an Israeli military spokesman, did not comment on the explosion at the plant, but told The Associated Press that Israel's latest strikes signal "a gradual increase in the pressure" on Hamas.
Israel is "determined to strike this organization and relieve us of this threat," Lerner said.
International calls for an unconditional cease-fire have been mounting in recent days, as the extent of the destruction in Gaza became more apparent.
The house of the mayor of the Bureij in central Gaza was hit in an airstrike, and five bodies were pulled from the rubble, the Red Crescent said. Those killed included the mayor, 50-year-old Anas Abu Shamaleh, his 70-year-old father and three relatives.
In the southern town of Rafah, seven members of one family were killed in an airstrike and seven members of a second family were killed when tank shells hit their home, according to the Rafah office of the Palestinian Center for Human Rights, which keeps a casualty count.
In central Gaza, seven people, including five members of one family, where killed by tank shelling on a home, the Red Crescent said.
Tens of thousands of Gazans have been displaced by fighting in the border areas, which have come under heavy tank fire. Late Monday, Israel urged residents of three large neighborhoods in northeastern Gaza to leave their homes and immediate head to Gaza City.
In the West Bank, Yasser Abed Rabbo, the secretary general of the Palestine Liberation Organization, called for a 24-hour cease-fire and said the offer was made after consultations with Hamas and the smaller Islamic Jihad group.
However, Izzat Rishq, a senior Hamas official in exile, said his group wanted to hear from Israel first.
The largest group in the PLO is the Fatah movement of Palestinian President Mahmoud Abbas, Hamas' main political rival. Hamas is not a member.
Despite appeals for a cease-fire, both sides have been holding out for bigger gains.
Hamas has said it will not stop fighting until it wins international guarantees that a crippling border blockade of Gaza will be lifted. Israel and Egypt had imposed the closure after Hamas seized Gaza in 2007, defeating forces loyal to Abbas. Over the past year, Egypt has further tightened restrictions, shutting down hundreds of smuggling tunnels under the Egypt-Gaza border that had provide crucial tax income to Hamas. The closure of the tunnels drove Hamas into a severe financial crisis.
Israel has said it is defending its citizens against attack from Gaza by hitting Hamas rocket launchers, weapons storage sites and military tunnels under the Gaza-Israel border.
Israel said its troops will not leave Gaza until they have demolished the tunnels which have been used by Hamas to sneak into Israel to try to carry out attacks.
Enav reported from Jerusalem. Associated Press writer Ibrahim Barzak in Gaza City contributed to this report.
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