US President Joe Biden declined to directly answer a question on reports that Israel was pumping seawater into Hamas’ Gaza tunnel complex, referring only to assertions that there were no hostages in the areas targeted.

Citing unnamed US officials, the Wall Street Journal reported that Israel had recently begun pumping seawater into Hamas’ vast labyrinth of tunnels underneath Gaza, in a process that would likely take weeks.

ABC News later published a similar report and said the flooding appeared limited as Israel evaluates the strategy’s effectiveness.

Israel’s military said it was looking into the reports. An Israeli defense ministry spokesperson declined to comment.

Responding to a question about the reports at a press conference in Washington, Biden said: “With regard to the flooding of the tunnels. I’m not at lib-, well. There (are) assertions being made that ... there’s no hostages in any of these tunnels. But I don’t know that for a fact.”

Biden added: “I do know that, though, every civilian death is an absolute tragedy, and Israel has stated its intent, as I said, to match its words ... with actions.”

The Journal, citing Biden administration officials, has said the flooding could help destroy the tunnels, where Israel believes the militant group is hiding hostages, fighters and munitions.

Other officials have expressed concerns the seawater would endanger Gaza’s fresh water supply, the Journal added.