The two top floors of the main building collapsed, and the second building, which provides housing for ministry employees, was set on fire, witnesses said.

Israeli aircraft also fired missiles at a Hamas camp in southern Gaza and a Hamas-affiliated school in Gaza City, and shelled open areas in the north. A 13-year-old boy was seriously wounded in the head by shrapnel, Palestinian medical officials said.

The rocket fired by Gaza militants Tuesday flew 7 miles and exploded in the courtyard of a school in Ashkelon, a city of 110,000 on Israel's seacoast north of Gaza. School security cameras showed a large cloud of white dust rising from the point of impact. The school was empty at the time and no one was hurt.

Although militants have launched many of the small, homemade rockets in the direction of Ashkelon, this was the first one to hit the center of the city. Militants have repeatedly claimed they were developing rockets with longer ranges.

Zeev Schiff, veteran military analyst for the respected Israeli Haaretz daily, wrote that the rocket attack on Ashkelon was "an unequivocal Hamas invitation to war."

In the hours before the rocket attack, Israeli forces were already operating in northern Gaza looking for tunnels, explosives, weapons warehouses and other facilities used by militants, according to the area army commander.

In a sign militant groups were stepping up their activity, soldier caught a Palestinian militant in the West Bank settlement of Barkan on Wednesday with an explosives belt strapped to his waist and ready for use, the army said. The militant was caught before he had a chance to detonate his explosives, the army said.

On Tuesday, Olmert ignored a deadline to free Palestinian prisoners set by the Gaza militants holding 19-year-old Israeli Cpl. Gilad Shalit.

A spokesman for the Army of Islam, one of the three groups that abducted Shalit, said the militants would not release any information about the soldier until Israel complies with the demands. However, he said there were no plans to kill the soldier.

Olmert has refused to negotiate with the militants. On Tuesday, he said he ordered the army "to strike terrorists and those who sent them and those who sponsor them ... None of them will be immune."

The threat was clearly meant for Syria, which harbors Hamas' top leader, Khaled Mashaal. Israel holds Syria responsible for Shalit's abduction, saying that Mashaal ordered the kidnapping. Last week, Israeli warplanes buzzed Syrian President Bashar Assad's palace.

In Gaza City, Palestinian Prime Minister Ismail Haniyeh of Hamas publicly called on the militants to protect the soldier. "The government is exerting efforts with Palestinian, Arab and regional parties to end this case in the appropriate manner," Haniyeh said at a Cabinet meeting.