U.S. Secretary of State Rex Tillerson warned Lebanon on Thursday that the Iran-backed Hezbollah group's growing arsenal and involvement in regional conflicts threatened Lebanon's security.

Speaking alongside Lebanese Prime Minister Saad al-Hariri in a Beirut news conference, Tillerson also said the United States was engaging with Lebanon and Israel to ensure their border remained calm.

Israel regards Hezbollah as the biggest direct threat on its borders and has repeatedly struck it in Syria, where the group is fighting alongside President Bashar Assad in a civil war now entering its eighth year.