The UN refugee agency says its Gaza headquarters has been struck by Israeli artillery fire and the building is now ablaze.
Spokesman Chris Gunness says the building was hit by what was believed to be three white phosphorous shells. The weapons burn at extremely high temperatures and can set things on fire.
However, witnesses said a nearby building was struck, and the UN building remained intact. It was hard to verify the accounts because the entire area was covered in black smoke.
Gunness says the building had been used as a shelter for hundreds of people fleeing Israel's 20-day offensive in Gaza. It's not clear how many people were there at the time. He says three people were injured.
The Israeli army had no immediate comment on the incident.
Meanwhile, an explosion blasted a tower block in the city of Gaza today that houses the offices of Reuters and several other media organisations, injuring a journalist for the Abu Dhabi television channel.
Reuters journalists working at the time said it appeared the southern side of the 13th floor of the Al-Shurouq Tower in the city centre had been struck by an Israeli missile or shell. Reuters evacuated the bureau.
Colleagues said at least one journalist working for Abu Dhabi television on the 14th floor was injured.
The 13th floor houses a local television production company. The Reuters bureau is on the 12th floor. Live television footage showed smoke coming from the roof of the building, which towers over the generally low-rise city centre.
An Israeli army spokesman said he was checking the report.