The Israeli army said the airstrike had targeted a Palestinian rocket squad. Israel has promised a harsh response to repeated rocket fire in recent days by Hamas militants.

Earlier Thursday, an Israeli airstrike destroyed a Hamas compound in central Gaza City, killing one and wounding at least 45 people, Palestinian witnesses and medical officials said.

The strike came a day after 22 Palestinians were killed across Gaza City in the most widespread factional fighting in nearly a year between Hamas and Fatah. The raging street battles have turned the densely populated seaside city into a war zone, putting terrified civilians increasingly at risk.

The army confirmed it carried out the airstrike, which came after Israel threatened "harsh" action in response to repeated Palestinian rocket attacks.

The explosion sent heavy plumes of smoke into the Gaza sky, completely destroying the structure and several others around it and sending terrified residents scurrying.

Hamas said the target was an administration building of its elite bodyguards unit. The two-story structure is normally filled with Hamas personnel.

In a text message sent to reporters, Hamas military wing spokesman Abu Obaida vowed revenge against Israel, possibly by suicide bombing.

"The Zionist enemy are launching an open war against Hamas. Therefore, reprisal options are open, including self-sacrifice operations," he said. "We advise the Zionist settlers to go immediately to the basement of their residence because our rockets will not spare any of them."

Palestinian President Mahmoud Abbas postponed a planned visit to the Gaza Strip after the latest violence in the area, officials said.

"The reason for the trip is to stop the fighting," presidential aide Saeb Erekat said, adding Abbas was determined to travel to Gaza in the next day or two.