JERUSALEM — Israeli jets and helicopters bombarded Gaza today and Hamas responded with a barrage of rockets on two cities as both sides defied a U.N. call for an immediate cease-fire.

One Israeli airstrike killed two Hamas militants and another unidentified man, while another flattened a five-story building in northern Gaza, killing at least seven people, including an infant, Hamas officials said. Israeli aircraft struck more than 30 targets before dawn, and there were constant explosions after first light.

By midday, 19 Palestinians had been killed, pushing the death toll to more than 760 and in the two-week-old conflict, according to Gaza health officials who say at least half of those killed were civilians. Thirteen Israelis have also been killed.

A U.N. Security Council resolution approved Thursday night called urgently for an immediate, durable and fully respected cease-fire, leading to the full withdrawal of Israeli forces from Gaza. The U.S., Israel's closest ally and a veto-wielding member of the Security Council, abstained.

While the call is tantamount to a demand on the parties, Israel's troops won't be required to pull out of Gaza until there is a durable cease-fire.

The resolution calls on U.N. member states to intensify efforts to provide guarantees in Gaza to sustain a lasting truce, including prevention of illicit trafficking in arms and ammunition.

In Israel's first official response to the resolution, Prime Minister Ehud Olmert's office said the Hamas rockets fired at Israel today "only prove that the U.N.'s decision is not practical and will not be kept in practice by the Palestinian murder organizations."

A Hamas spokesman said the Islamic militant group "is not interested" in the cease-fire because it was not consulted and the resolution did not meet its minimum demands.

Israel launched its assault on Dec. 27 in an attempt to halt years of rocket fire from the Hamas-controlled territory.

Despite the devastating offensive, Hamas has kept up rocket attacks on southern Israel. The rockets fired today hit in and around two of the largest southern cities, Beersheba and Ashkelon. Cities within rocket range of Gaza have largely been paralyzed since the fighting began.