Gaza health officials say nine people, including three children, have died in a blast in the northern Gaza Strip.

The cause of the blast was not immediately known.

The deaths came as Hamas militants were firing rockets into Israel on Monday, but it remains unclear if the victims were hit by an errant rocket attack or an Israeli reprisal.

An Israeli military spokesman said Israel had started to attack military targets in Gaza and added more troops to its Gaza division.

The Israelie military also said Hamas will "bear the consequences" for the rocket attack.

Hamas had earlier claimed responsibility for that attack, in which a rocket hit a civilian vehicle leaving one person injured, according to the Israeli military.

Rocket-warning sirens had sounded in Jerusalem and several explosions were heard nearby, minutes after an ultimatum from Gaza's ruling Hamas militant group expired.

Hamas had set a deadline for Israel to remove its security forces from the Al-Aqsa Mosque compound, where Israeli police had earlier used stun grenades and tear gas against stone-throwing Palestinians.

Abu Obeida, spokesman for Hamas' military wing, said the rocket attack was a response to what he called Israeli "crimes and aggression" in Jerusalem.

The Israeli Army said there was an initial burst of seven rockets, one of which was intercepted.

The early-evening attack escalated already heightened tensions throughout the region following weeks of clashes between Israeli police and Palestinian protesters in Jerusalem.

Hundreds of Jewish worshippers were evacuated from Jerusalem's Western Wall on Monday, following the rocket fire from Gaza.

"An alarm has just been sounded in Jerusalem. Police forces have begun evacuating hundreds of people" gathered at the Wailing Wall to safer locations, police said in a brief statement.

Reuters/AP/ABC