Israel opened border crossings with the Hamas-run Gaza Strip on Monday, allowing in limited amounts of food and fuel for the second time in three weeks after the United Nations warned of a looming humanitarian crisis.

Aid groups said the one-day shipment would have minimal impact because border crossings have been closed for so long, depleting reserves of everything from flour to animal feed.

"It is just not enough," said Christopher Gunness, spokesman for the United Nations Relief and Works Agency (UNRWA).

Israel agreed to let in 45 truckloads of goods through the Kerem Shalom crossing, including 10 for UNRWA, officials said. Gunness said his agency needed about 15 trucks a day.

Some 15 truckloads of grain and animal feed went through the Karni crossing for the first time in three weeks, Palestinian officials said.

Israeli Defence Minister Ehud Barak ordered the crossings to open after a drop in the number of cross-border rockets fired by Palestinian militants from the Gaza Strip.

Israel clamped down on humanitarian imports to the Gaza Strip starting on November 4, when a deadly army raid into the coastal territory triggered a surge in rocket attacks.

Kerem Shalom was last opened on November 17 to let in about 30 truckloads of goods.

For the first time since November 12, Israel also allowed in European Union-funded fuel to the Gaza Strip's sole power plant, a move that could temporarily alleviate chronic blackouts.

Hamas Health Minister Basim Naeem said Gaza faced a "real crisis that cannot be alleviated by these Zionist tricks," referring to the limited number of trucks allowed in.

Gunness said UNRWA cannot function normally without a steady stream of supplies, not only food but books for schoolchildren, also blocked by Israel for weeks.

A Palestinian official said Israel agreed to reopen the crossings following mediation by Egypt, which prevailed on Gaza militants to simultaneously stop firing rockets.

Israeli officials said future aid shipments hinged on keeping a lid on border violence which has disrupted a 5-month-old truce, due to expire in December, along the Israel-Gaza frontier.

Both Israel and Hamas have said they want to restore the Egyptian-brokered ceasefire, which calls on Hamas to halt rocket fire and other attacks against the Jewish state.

The ceasefire also demands Israel gradually ease the blockade it tightened on the Gaza Strip more than a year ago after Hamas Islamists routed President Mahmoud Abbas's secular Fatah forces.