WASHINGTON - The Syrian Democratic Forces (SDF), supported by the U.S.-led coalition against the Islamic State, say they believe major IS leaders are hiding on the Iraq-Syria border amid an ongoing operation to recapture the last Islamist strongholds.

The U.S.-backed operation, code-named al-Jazeera Storm, was launched last week to remove IS from the rest of the territory it still controls in Deir el-Zour province in eastern Syria.

The forces say the area contains hundreds of IS remnants and key leaders, foreign and local, who took shelter in the vast desert terrain border area after surviving the intense battles in Mosul and the self-proclaimed capital Raqqa.

“Powerful military figures of IS, including international terrorists, are hiding in the region,” said Abu Khawla, the commander of Deir el-Zour Military Council.

He said the fate of the IS leaders will be either “capture or death” as the militants face a siege from the Iraqi army and allied Shiite militias on the Iraqi side and an advancing U.S.-backed operation on the Syrian side.

“We are watching them through intelligence as the battle continues,” he told VOA.

WATCH: Syrian Democratic Forces Target Major Islamic State Leaders

The U.S.-backed SDF in the past weeks has caught several IS leaders in Deir el-Zour as it closed in on the group. Last month, the force announced it detained German militant and the suspected September 11 recruiter Mohammed Haydar Zammar in northern Syria.

Abu Adil, a member of SDF’s counterterrorism unit who took part in the operation to capture Zammar, told VOA that the fighter was hiding in a compound in eastern Deir el-Zour which was surrounded during a raid that took several hours. “He surrendered to us after about one hour of negotiation,” said Adil.

Zammar was held in a prison in Raqqa and investigated for seven days before being handed to the SDF Public Security Forces, according to Adil. He said Zammar provided information about several terrorist attacks he engineered and two meetings he had with former al-QaIda leader Osama bin Laden.

The U.S.-led coalition and local partners have been targeting IS senior personnel in Iraq and Syria since 2014 to disrupt the organization’s structure and ability to recruit and train members. The coalition airstrikes in 2016 killed the IS spokesman Abu Mohammad al-Adnani and IS chief commander Abu Omar al-Shishani.

FILE - This image made from video posted on a mili FILE - This image made from video posted on a militant website July 5, 2014, purports to show the leader of the Islamic State group, Abu Bakr al-Baghdadi, delivering a sermon at a mosque in Iraq. FILE - This image made from video posted on a militant website July 5, 2014, purports to show the leader of the Islamic State group, Abu Bakr al-Baghdadi, delivering a sermon at a mosque in Iraq.

But top IS leader Abu Bakr al-Baghdadi, the world's most wanted man, with a $25 million bounty on his head, has evaded capture.

The SDF leaders who spoke to VOA refused to comment on whether al-Baghdadi was in the list of IS leaders they suspect to be sheltering in eastern Syria.

The IS head has been declared injured or dead several times in the past, including reports that he was hit by a Russian strike in 2017. Those claims have been rejected and the whereabouts of the leader remain unknown.

Several Iraqi officials this week declared that al-Baghdadi was likely hiding in villages on the Euphrates River Valley, near the Iraqi border.

“The last information we have is he is in Al-Hajin in Syria, 18 miles from the border in [Deir el-Zour] province,” Abu Ali al-Basri, director-general of the intelligence and counter-terrorism office at the Iraqi Ministry of Interior, told Fox News on Sunday.

Al-Basri said the information is being used to conduct a “multi-force raid” with Russian, Syrian and Iranian troops.

Zana Omar contributed to this report from Dier el-Zour, Syria.