CAIRO (Reuters) - Talks involving Qatari, Egyptian and U.S. mediators aimed at reaching a ceasefire between Israel and Hamas in the Gaza war were still underway on Thursday but had shown no sign of a breakthrough, two Egyptian security sources said.

The talks began on Wednesday, when CIA director William Burns met senior officials from Qatar and Egypt in Doha to discuss a proposal that U.S. President Joe Biden publicly endorsed last week. Biden described the three-phase plan as an Israeli initiative.

The talks in Qatar were aimed at finding a formula that could reassure Hamas over its demand for guarantees that the deal would deliver a complete cessation of hostilities in the Gaza Strip and a full Israeli withdrawal from the territory, the Egyptian sources said.

Hamas expressed concerns about some provisions of the proposal, especially the second phase, the sources added.

According to a summary of the plan published by the White House, the second phase includes a permanent end to hostilities as well as the withdrawal of Israeli forces from Gaza.

The Egyptian sources said that Qatari and Egyptian mediators had met separately with Hamas and U.S. officials in Doha. They said there was no indication a deal was close to being reached.

Qatari, Egyptian and U.S. officials have been holding negotiations for months aimed at securing a ceasefire in Gaza as well as the release of Israeli hostages and Palestinian prisoners.

Hamas leader Ismail Haniyeh said on Wednesday that the group would "deal seriously and positively with any agreement that is based on a comprehensive ending of the aggression and the complete withdrawal and prisoners swap".

Israel said there would be no halt to fighting during ceasefire talks as it mounted a new assault on a central section of the Gaza Strip.

Senior Hamas official Sami Abu Zuhri told Reuters on Thursday that while the group welcomed what he called "Biden's ideas", the U.S. draft resolution at the U.N. Security Council was dependent on an Israeli ceasefire proposal Hamas had seen and had rejected.

"The (U.S.) document...has no mention of ending the aggression or the withdrawal," he said.

"The Israeli documents speak of open-ended negotiation with no deadline, and it speaks of a stage during which the occupation regains its hostages and resumes the war. We had told the mediators that such a paper wasn't acceptable to us," said Abu Zuhri.

He said Hamas was committed to its May 5 proposal which was was based on an end to the fighting and an Israeli withdrawal, a swap deal, and a lifting of the blockade of the enclave.

The war began after Hamas attacked Israel on Oct. 7, killing around 1,200 people and capturing more than 250 hostages, according to Israeli tallies.

Israel's military response in Gaza has killed more than 36,000 people, according to Gaza health officials, who say thousands more are feared buried under the rubble.

(Reporting by Ahmed Mohamed Hassan in Cairo, Writing by Aidan Lewis, Editing by Angus MacSwan)