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JERUSALEM, Jan 15 (Reuters) - United Nations Secretary General Ban Ki-moon said on Thursday it might take some days to work out the technical details of a ceasefire in the Gaza Strip but urged Israel and Hamas to stop fighting immediately.

"It may take a few more days to agree to a few more technical issues," Ban told reporters after a series of meetings with Israeli leaders. He added that he was "reasonably optimistic" the Israelis would decide to accept a ceasefire.

Ban said he discussed Gaza in telephone calls with Egyptian President Hosni Mubarak, who has been negotiating a truce with Israel and Hamas, as well as U.S. Secretary of State Condoleezza Rice and British Prime Minister Gordon Brown.

"(Mubarak) thought that they were they were going very close to a conclusion, to a ceasefire," Ban said. "But there seem to be some other detailed technical issues which need to be further elaborated and agreed."

But he urged both Israel and Hamas to stop their fighting before any such technical details are worked out.

"They first should stop the fighting, agree to an immediate ceasefire and continue their negotiations," Ban said.

The issue of how long the ceasefire would last, he said, did not seem to be a problem. More difficult were issues such as preventing the illegal smuggling of weapons into Gaza, he said.

"I don't think the duration of ceasefire is a serious problem at this time," he said.

Ban was also asked what he thought of Israeli suggestions that Palestinian militants may have been firing at Israeli soldiers from the roof of the U.N. aid compound in Gaza, which Israeli forces shelled on Thursday.

The U.N. chief dismissed the idea, saying, "These cannot be convincing explanations."

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