In Thursday's talks with Lebanese Prime Minister Saad Hariri in Beirut, the secretary of state offered Washington's support for a "free and democratic" Lebanon.
It is a key ally of Lebanon's President, Michel Aoun, and of Syrian President Bashar al-Assad.
Lebanon is a big recipient of USA military support. Lebanon says part of the wall follows the UN-demarcated "Blue Line" that was drawn up after Israel withdrew from southern Lebanon in 2000, and insists some sections will cut into its territory.
On Wednesday, while still on the Jordanian leg of his Middle East tour, Tillerson had struck a softer tone towards the organization, conceding that Hezbollah was part of the "political process" in Lebanon.
Hezbollah, which the United States deems a terrorist organisation, has increased its military strength since the last major conflict with Israel in 2006. It has fighters on the ground in Syria supporting Assad's regime.
Visiting U.S. Secretary of State Rex Tillerson has reaffirmed the importance of the U.S. close partnership with Lebanon, the U.S. embassy said Thursday.More news: Snapchat down for many: Here's the official advice
A sudden shift has been appeared in the Lebanese politics after the PM Saad Hariri was reportedly detained in Saudi Arabia and forcefully made statements against the Hezbollah.
Lebanon has been forced to deny that visiting US Secretary of State Rex Tillerson was made to wait ahead of a meeting with President Michel Aoun on goal, a departure from diplomatic protocol. "The aid provided by the worldwide community is insufficient and is already going directly to the displaced without any coordination with or supervision by the Lebanese State", Aoun said during talks with US Secretary of State Rex Tillerson, as quoted by the National News Agency (NNA).
The U.S.is working with the Lebanese and Israeli governments to maintain calm in Lebanon's south, the U.S. minister said during the visit. Lebanese authorities regularly call on the global community to assist them in handling the refugee crisis, as the country has the highest per capita concentration of refugees worldwide.
Israel has recently escalated its threats over Lebanon's invitation for offshore gas exploration bids along the disputed maritime border.
Lebanon is also protesting Israeli violations of Israeli airspace, often to bomb targets inside neighboring Syria.
Earlier this month, Lebanese military officials complained about the wall impinging on Lebanese soil when they met with their Israeli counterparts at a regular tripartite meeting chaired by United Nations peacekeepers in Ras al-Naqoura, near the border. Hezbollah's entanglement in regional conflicts "threatens the security of Lebanon and has destabilizing effects in the region", he added.