The Israeli military has begun pumping seawater into Hamas' tunnel complex in Gaza, the Wall Street Journal reported on Tuesday citing unnamed US officials, adding that the process would likely take weeks.

Some Biden administration officials have said the process could help destroy the tunnels, where Israel believes the militant group is hiding hostages, fighters and munitions, the Journal reported. Other officials have expressed concerns the seawater would endanger Gaza's fresh water supply, the newspaper reported.

Israel's military did not immediately provide comment on the report. An Israeli defence ministry spokesperson did not immediately respond to a request for comment. President Joe Biden declined to directly answer a question on the reports, referring only to assertions that there were no hostages in the areas targeted.

Responding to a question, 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."