He made the remarks at the one-day annual Arab Strategy Forum in Dubai, Xinhua news agency reported.
"As Hezbollah had helped the Syrian regime defeat the Islamic State (IS) group, this might encourage the militia to step up attacks on Israel," said Gates, who served under US Presidents George W. Bush and Barack Obama.
Israel in return might feel that Hezbollah has become a problem to deal with, he added.
Hezbollah has been supporting the Syrian regime of President Bashar al-Assad in its fight against the IS and other militia groups in Syria by sending fighters and weapons.
In 2006, Hezbollah and Israel fought a 34-day war and have since exchanged sporadic fire along the Lebanese-Israeli border.
Gates said he expects the tension to rise between Shia Iran and Saudi-led Sunni Arab states "as Iran continues in trying to increase its influence and meddling across the Middle East".
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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