By Erin Banco

NEW YORK (Reuters) - The Palestinian militant group Hamas has recruited between 10,000 and 15,000 members since the start of its war with Israel, according to two congressional sources briefed on U.S. intelligence, suggesting the Iran-backed fighters could remain a persistent threat to Israel.

The intelligence indicates a similar number of Hamas fighters have been killed during that period, the sources said. The latest official U.S. estimates have not been previously reported.

Hamas and Israel began a ceasefire on Sunday after 15 months of a conflict that has devastated the Gaza Strip and inflamed the Middle East.

The sources briefed on the intelligence, which was included in a series of updates from U.S. intelligence agencies in the final weeks of the Biden administration, said that while Hamas has successfully recruited new members, many are young and untrained and are being used for simple security purposes.

The U.S. Office of the Director of National Intelligence declined to comment.

On Jan. 14, then-President Joe Biden's Secretary of State Antony Blinken said the United States believed Hamas had recruited almost as many fighters as it had lost in the Palestinian enclave, cautioning that this was a "recipe for an enduring insurgency and perpetual war."

He did not provide further details about the assessment, but Israeli figures have put the total militant death toll in Gaza at around 20,000.

“Each time Israel completes its military operations and pulls back, Hamas militants regroup and re-emerge because there’s nothing else to fill the void,” Blinken said. Both Israel and the United States brand Hamas a terrorist group.

Asked for comment, a Hamas official said he was checking with the relevant parties in the group. Hamas armed wing spokesman Abu Ubaida said in July that the group had been able to recruit thousands of new fighters.

In the days since the ceasefire, Hamas has shown itself to be deeply entrenched in Gaza despite Israel's vow to destroy the militant group. The territory's Hamas-run administration has moved quickly to reimpose security measures and to begin restoring basic services to parts of the enclave, much of which has been reduced to wasteland by the Israeli offensive.

Since the start of the war, American officials have not said publicly how many fighters Washington believes Hamas has lost, only noting that the group has been significantly degraded and has likely lost thousands.

WARNINGS OF A CONTINUED THREAT

U.S. officials have issued similar warnings since Hamas' Oct. 7, 2023, attack on Israel in which 1,200 people were killed and more than 250 were taken hostage, according to Israeli tallies. More than 46,000 people have been killed in the Israeli assault that followed, according to Palestinian health authorities whose figures do not differentiate between civilians and combatants.

At a congressional hearing in March 2024, then-Director of National Intelligence Avril Haines said that the war in Gaza would have "generational impact on terrorism" and that the crisis had already "galvanized violence by a range of actors around the world."

Gathering exact data on Hamas is notoriously difficult because of a lack of verifiable intelligence from inside Gaza and because the group's recruitment and training efforts are fluid. But official U.S. figures show that prior to Oct. 7, 2023, Hamas had anywhere between 20,000 and 25,000 fighters.

Asked on Wednesday about Blinken's comments, Israel's U.N. Ambassador Danny Danon acknowledged Hamas' recruitment efforts but played down the threat.

"We know that Hamas recruits youngsters," Danon said. "But even if they recruit youngsters, they don't have the weapons or the training facilities. So basically, yes, you can incite those youngsters against Israel, but they cannot become a terrorist, because you cannot equip them with weapons or rockets."

Following the ceasefire, Israeli troops have begun to move back from some of their positions inside Gaza. The second phase of the ceasefire deal could bring about a permanent end to the fighting.

The terms of that phase still need to be negotiated.

In his resignation speech on Tuesday, Lieutenant General Herzi Halevi, Israel's military chief, said Hamas had been severely damaged and that most of the group's military commanders had been killed. But he said the group had not been eliminated and the Israel Defense Forces would continue to fight to further dismantle Hamas.

One of the most difficult issues involved in negotiating the next phases is postwar Gaza's governance. Some Israeli officials say they won't accept Hamas staying in power. Hamas so far has not given ground.

Newly inaugurated President Donald Trump's national security adviser Mike Waltz said on Sunday that Hamas will never govern Gaza and if it reneges on the deal, Washington will support Israel "in doing what it has to do."

(Reporting by Erin Banco; Additional reporting by Nidal al-Mughrabi; Editing by Don Durfee and Howard Goller)