Gen Petraeus will soon take charge of all US military operations in the region
The US commander in Iraq, Gen David Petraeus, has held talks with Lebanese President Michel Suleiman and other leaders on a surprise visit to Beirut.
His talks with army chiefs focused on improving training, logistics and the defence capabilities of Lebanon's army, a statement said.
Gen Petraeus has been appointed to head the US Central Command, overseeing US military operations in the Middle East.
He has accused Lebanon's Hezbollah group of helping Shia militias in Iraq.
The US has sharply increased military aid to Lebanon since the 2006 war between Hezbollah and Israel, giving it $270m in 2007.
Correspondents say the aid is meant to show support to Prime Minister Fouad Siniora's Western-backed government, which is under pressure from the Shia militant Hezbollah and its allies in parliament.
After meeting acting army commander Maj Gen Shawki al-Masri, Gen Petraeus met Mr Siniora for more than an hour. Gen Petraeus is due to take command of Central Command in September.
A long political crisis in Lebanon was eased in May when Mr Siniora formed a new government in which the Hezbollah-led alliance had veto power over cabinet decisions.