Israel is to halt its three-week military offensive against Hamas militants in the Gaza Strip, Prime Minister Ehud Olmert has said.

He said Israel had achieved its aims and the unilateral ceasefire would start at 0200 (2400 GMT). But he said troops would remain in Gaza for now.

A Hamas spokesman said it would not accept one Israeli soldier in Gaza.

Nearly 1,200 Palestinians have been killed since the violence began on 27 December. Thirteen Israelis have died.

Minutes before Mr Olmert was due to speak, a rocket was fired from Gaza, a BBC producer in Gaza said.

'Goals achieved'

The Israeli prime minister's announcement came in a televised address following a late-night cabinet meeting.

'Relentless bombardment'

In an interview with the BBC, Mr Ging, director of operations in Gaza for the UN Relief and Works Agency (Unrwa), said the area surrounding its compound had been under "relentless artillery and tank bombardment all night and all day".

Some rounds, Mr Ging said, had struck a part of the compound where about 700 residents of nearby blocks of flats were taking shelter. Three people were injured in the bombardment.

"Then an hour later, in spite of our protests and real-time discussions with the Israeli liaison, three rounds that emitted phosphorous struck the other corner of the compound," he added.

The compound is Unrwa's main distribution hub in Gaza and Mr Ging said the shells set alight part of a warehouse in which there were stored thousands of tonnes of food and medicine, and the workshop area.

The fires then threatened to engulf five fuel tankers, which had been due to be sent out that morning, but could not leave because it was too dangerous outside.

"When the fires broke out, five of our brave staff dashed down there and moved the trucks out of the area, so we avoided a massive explosion," he said.

Mr Ging told CNN the fire was very difficult to extinguish because the smoke from WP becomes toxic if water is used.

Following the incident, UN Secretary General Ban Ki-moon - in Israel to push for a ceasefire in the Gaza Strip - expressed his "outrage" and demanded a full explanation from the Israeli government.

"The defence minister said to me it was a grave mistake and he took it very seriously. He assured me that extra attention will be paid to UN facilities and staff and this will not be repeated," Mr Ban said.

Israeli Prime Minister Ehud Olmert said the compound had been targeted after militants had opened fire from there.

"It is absolutely true that we were attacked from that place, but the consequences are very sad and we apologise for it," he said. "I don't think it should have happened and I'm very sorry."

Key Hamas leader killed in Gaza

One of the top Hamas leaders in Gaza has been killed in an air strike, Hamas and Israeli officials have said.

Said Siyam, the Hamas interior minister, was killed in an air raid on his brother's home near Gaza City.