(CNN) – US President Joe Biden on Tuesday commented on reports that Israel has begun filling some tunnels in the Gaza Strip with seawater to destroy a network of tunnels used by the Hamas movement.

Biden said during a press conference with his Ukrainian counterpart, Volodymyr Zelensky, that “although he has asked for assurances that there are no hostages currently in those tunnels, the administration has not been able to confirm this beyond a reasonable doubt, but the death of a citizen I know is an absolute tragedy.”

A U.S. official told CNN that the Israelis had told the U.S. that they had begun conducting “detailed tests” to bring seawater into some tunnels in the Gaza Strip “at low levels” to see if it could disrupt the tunnel network. On a large scale.

The official added that the Israelis “are not yet sure whether the plan will succeed or not, but they have assured the US that they are only interested in testing it in tunnels where they do not believe there are hostages.”

CNN reached out to the Israeli military for comment.

Israeli Prime Minister Benjamin Netanyahu’s office said earlier that 135 hostages were still being held by Hamas.

The Israeli military announced earlier this month that it had “destroyed at least 500 tunnel openings in Gaza and surrounded more than 800 of them”.

Biden said during the press conference that he held talks with Netanyahu “to focus on protecting civilian lives as much as possible, even after the atrocities committed in the October 7 Hamas attack.”

He added: “I think we have made the matter clear to the Israelis, and they realize that their actions must be consistent with trying to do everything possible to prevent innocent Palestinian civilians from being harmed, killed, killed, lost.”

He pointed to his administration’s efforts to ensure the delivery of humanitarian aid to Gaza, and praised Israel and Arab countries for providing aid, including food and fuel.

Biden announced that two senior U.S. officials will travel to Israel this week for important meetings, namely National Security Adviser Jack Sullivan, where he will meet with the Israeli war cabinet to “stress our commitment to Israel and the need to protect civilian lives. And to ensure the flow of humanitarian aid and access to Gaza for Palestinian civilians.” , Secretary of Defense Lloyd Austin will visit the region to “intensify international efforts to protect trade barriers through the Red Sea.”