JERUSALEM — The United Nations halted aid deliveries to the besieged Gaza Strip on Thursday, citing Israeli attacks on its staff and installations hours after it said tank fire killed one of its drivers as he went to pick up a shipment.

The United Nations has already demanded an investigation into Israel's shelling of a U.N. school in Gaza that killed nearly 40 people earlier this week. Israel and residents said militants were operating in the area at the time.

For a second straight day, Israel suspended its Gaza military operation for three hours to allow in humanitarian supplies. Shortly before the pause took effect, however, the U.N. said one of its aid trucks came under Israeli fire, killing the driver.

U.N. spokesman Adnan Abu Hasna said the U.N. coordinated the delivery with Israel, and the vehicle was marked with a U.N. flag and insignia when it was shot in northern Gaza. The Israeli army said it was investigating.

"The U.N. is suspending its aid operations in Gaza until we can get safety and security guarantees for our staff," spokesman Chris Gunness said. "We've been coordinating with them (Israeli forces) and yet our staff continue to be hit and killed."

The U.N. provides food aid to around 750,000 Gaza residents, and runs dozens of schools and clinics throughout the territory. They have some 9,000 locally-employed staffers inside Gaza, and a small team of international staffers who work there.

As Israel pushed forward with the bloody offensive in the Gaza Strip, militants in Lebanon fired at least three rockets into Israel early Thursday, threatening to open a new front for the Jewish state. Israel responded with mortar shells.

The rockets from Lebanon raised the specter of renewed hostilities on Israel's northern frontier, just 2 1/2 years after Israel battled the Hezbollah guerrilla group to a 34-day stalemate. War broke out between Hezbollah and Israel in 2006 as Israel battled Palestinian militants in Gaza, on Israel's southern borders.

No group claimed responsibility. Lebanon's government condemned the attack, and Hezbollah — which now plays an integral role in Lebanon's government — denied any responsibility for the rocket fire, which lightly injured two Israelis.

In other Gaza violence, Israel killed at least 11 people, including three who were fleeing their homes, raising the death toll from its 13-day offensive to 699 Palestinians, according to Palestinian medical officials. Eleven Israelis have died since the offensive began Dec. 27.