Diplomatic talks to end the fighting in Gaza as civilian casualties mount are "very complicated," a US official admitted Thursday, refusing to predict how long they could take.

US Secretary of State John Kerry has been in Cairo since Monday trying to persuade Israel and Hamas to lay down their arms in the 17-day war in which some 788 Palestinians have been killed along with 35 Israelis, most of them soldiers.

The top US diplomat visited both Israel and West Bank on Wednesday to meet with Israeli and Palestinian leaders, but his efforts have so far failed to produce any concrete results.

Asked if it might be possible to agree a ceasefire before the end of the Muslim holy month of Ramadan due early next week, deputy State Department spokeswoman Marie Harf said it was "a tough issue."

"We're on the ground working through these issues, but it is very complicated," she told reporters.

"I don't have predictions to make for how long this will take, but I think all you have to do is look at what's happening on the ground to see that this needs to happen as soon as possible."

She refused to get into specific details of what the framework of any eventual ceasefire might look like, other than to say that the US was backing an Egyptian initiative which was based on a 2012 truce which was violated almost immediately after it was put into place.

Hamas leader Khaled Meshaal, who is based in Qatar, has rejected that proposal saying the Islamic militant group which controls Gaza will not agree to a ceasefire until it has firm guarantees that the long-time Israeli blockade of the impoverished Strip will be lifted.

Hamas is blacklisted by Israel and the United States as a terrorist organization and conducts its negotiations through intermediaries such as Turkey and Qatar.

Kerry spoke three times by phone with Turkish Foreign Minister Ahmet Davutoglu on Thursday as well as with his Qatari and Jordanian counterparts and Israeli Prime Minister Benjamin Netanyahu.

US deputy national security adviser Tony Blinken has called for an agreement that would ultimately result in the demilitarization of the Gaza Strip to prevent further crises.

"One of the results, one would hope, of a ceasefire would be some form of demilitarization, so that again, this doesn't continue," Blinken told National Public Radio, but this again would seem to be a longer term goal that would need to be worked on after both sides lay down their arms.

*This story was edited by Ahram Online.