The UN has launched an appeal for $613m to help people affected by Israel's military offensive in Gaza.

"These needs are massive and multi-faceted," the body's top official Ban Ki-moon said at the World Economic Forum in Davos.

He added that funds would be used to "help overcome at least some measures of this hardship".

The announcement came amid fears of a collapse of unilateral ceasefires by Israel and the Hamas militant group.

Two rockets have been fired at Israel, while Israeli air strikes hit southern Gaza.

UN Secretary-General Mr Ban visited Gaza after the ceasefires - the first world leader to do so since Hamas took over in 2007 and Israel tightened its blockade - and he spoke passionately about what he had seen there.

"More than one-third of the 6,600 deaths and injured were children and women. As a father of three I was especially troubled by the suffering and trauma that so many families went through," he said.

"People have lost their families, they have lost their homes, belongings, and livelihoods. Schools, clinics, factories and businesses have been destroyed," he said.

Open crossings

BBC diplomatic correspondent Bridget Kendall in Davos says Mr Ban's appeal was impassioned and hard hitting, and he stressed it was humanitarian rather political in nature.

The appeal aims to cover the requirements of the UN and other aid organisations for the next six to nine months and will provide everything from medical care to clean water.

He told reporters that an appeal for longer-term needs would be launched later.

The Israeli offensive in Gaza killed about 1,300 Palestinians, of whom 412 were children; 21,000 homes were destroyed or badly damaged. Thirteen Israelis were killed during the three weeks of violence.

Also at Davos, the UN Office for the Co-ordination of Humanitarian Affairs chief John Holms said Israel had to lift its blockade of the Palestinian territory of 1.5 million people.

"We're asking for the crossing points to be opened fully... Otherwise, we won't be able to achieve what we want to achieve," he said at a joint news conference with Mr Ban.

Israel says restrictions on access to Gaza are necessary to prevent the flow of weapons to the Hamas movement.

Hamas boycott

In the region itself, George Mitchell, envoy for the new Obama administration in the US, travelled to the West Bank for a meeting with Palestinian Authority President Mahmoud Abbas.

On Wednesday, he met senior Israeli officials in Jerusalem, after beginning his first tour of the region as President Barack Obama's envoy in Egypt.

Mr Mitchell has no plans to meet representatives of Hamas.

Sporadic violence has erupted since both sides declared tentative ceasefires on 17 and 18 January, with an apparent surge in the last 36 hours.

The first rocket was fired from the Gaza Strip late on Wednesday and Israel has been bombing the area around the border between Gaza and Egypt. Another rocket was fired on Thursday morning.

Meanwhile, about 40 demonstrators protested outside the building housing the BBC offices in Gaza.

Representatives from a dozen intellectual and cultural groups called for a boycott of the BBC, in protest at its refusal to screen on UK television a humanitarian aid appeal for Gaza and for what they describe as "unbalanced reporting".