WASHINGTON 

US President Joe Biden said Tuesday that he cannot confirm whether any of the hostages taken by Hamas two months ago remain in a system of labyrinthine tunnels under Gaza as Israel begins pumping in sea water. 

"With regard to the flooding of the tunnels, there are assertions being made that they’re quite sure there are no hostages in any of these tunnels. But I don't know that for a fact," Biden said, apparently referring to assurances from Israel. He did not elaborate, however.

"I do know though that every civilian death is an absolute tragedy, and Israel has stated its intent, as I said, to match its intent with actions," he added.

Roughly 139 hostages remain in captivity following the Palestinian group Hamas's surprise Oct. 7 cross-border attack, according to official figures. Over 1,200 other people were killed.

The Wall Street Journal, citing US officials briefed on Israeli military operations, reported earlier Tuesday that Israel's decision to flood the tunnels with water from the Mediterranean Sea is part of a broader strategy being employed to destroy the tunnel network.

Hamas has used it to move fighters and arms covertly in the densely-populated coastal enclave. Israeli officials separately believe some hostages are being held there, the Journal reported. Hostages' families have told Israeli Prime Minister Benjamin Netanyahu that they fear their loved ones will be killed if the tunnels are flooded, according to reports.

The tunnel system extends for 300 miles (482 kilometers), and the use of thick blast doors is being assessed by the Israelis, according to US officials. The process is expected to take several weeks to complete.

The move, which was first announced by the newspaper earlier this month, has drawn criticism, with some saying it would create an environmental catastrophe by contaminating Gaza's already limited fresh water supplies and significantly harm local agriculture.

Israel has bombarded the Gaza Strip from the air and land, imposed a siege and mounted a ground offensive in retaliation for a cross-border attack by Hamas on Oct. 7.

At least 18,412 Palestinians have been killed and 50,100 injured in the Israeli onslaught since then, according to Gaza’s health authorities.