Israel’s military has started pumping seawater into Hamas’ extensive tunnel system underneath the Gaza Strip as part of an effort to drive terrorists out of their hiding places, a report says. 

The tactic is unfolding Wednesday in its early stages, U.S. officials briefed on Israel’s military operations told The Wall Street Journal. 

They said flooding the tunnels would likely be a weekslong process and Israel began pumping seawater from the Mediterranean after carrying out some testing and installing an additional two pumps alongside the five that were set up near the coastal Al-Shati refugee camp in Gaza City last month. 

It’s unclear how many of Hamas’ hostages may be in the tunnel network underneath Gaza. 

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On Tuesday, when asked about the flooding tactic, President Biden said during a press conference in Washington that assertions have been made that "there are no hostages in any of these tunnels… But I don’t know that for a fact," according to The Wall Street Journal. 

In a report published a week ago, American officials told the newspaper that Israel first informed the U.S. of the plan in early November, drawing a mixed reaction as there are concerns about how pumping seawater may damage the environment.

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"We are not sure how successful pumping will be since nobody knows the details of the tunnels and the ground around them," one source told The Wall Street Journal. "It’s impossible to know if that will be effective because we don’t know how seawater will drain in tunnels no one has been in before." 

Each pump is capable of drawing water from the Mediterranean Sea and can move thousands of cubic meters of water per hour, the report said. 

According to the report, Israel has identified about 800 tunnels beneath Gaza that Hamas has used to move fighters, store weapons and plan terror attacks on Israel. However, Israeli officials believe the tunnel network is much larger. 

Fox News’ Anders Hagstrom and Chris Pandolfo contributed to this report.