US Secretary of State Rex Tillerson said on Thursday that Lebanon's security was threatened by the growing arsenal of the Iran-backed group Hezbollah and its involvement in regional conflicts.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 the border remained calm.Tensions between the states have spiked recently because of Israeli plans to build a wall at the border, and because of Lebanon's decision to begin exploring for oil and gas in an offshore block in disputed waters.Shi'ite Hezbollah, which is deemed a terrorist organization by the United States, is part of Hariri's government. Its military strength has grown since the last major conflict with Israel in 2006.Tillerson urged Lebanese leaders to uphold the country's commitment to staying out of regional conflicts, a reference to Hezbollah's role in wars around the Middle East including in neighboring Syria where Iran also holds major sway.Israel has accused Iran of seeking to set up weapons factories in Lebanon, and the Israeli military last month said the country had turned into one "large missile factory."Lebanon is a big recipient of US military support. Tillerson said Washington remained committed to supporting the Lebanese army and internal security forces. The United States has been mediating between Lebanon and Israel over their border dispute. In apparent reference to the maritime dispute, Tillerson said: "We've asked no one to give up anything."He added that constructive discussions were underway and he urged the Israelis to be constructive.