Terje Roed-Larsen, the UN's Middle East envoy, speaking at a news conference on Sunday, said: "We don't believe that it is indeed possible to go down south or into the Bekaa Valley and take away the weapons of Hizbollah.
"Our goal is to integrate Hizbollah into the Lebanese army."
Roed-Larsen will present a report in April on progress in the implementation of the Security Council resolution 1559, which demands that foreign troops should leave Lebanon and militias there disarm.
His comments came at the end of a 20-day tour that took the Norwegian diplomat around Arab capitals as well as to Paris, Washington, London, Moscow and Beijing to discuss Hizbollah's weapons and the armed Palestinian factions based in Lebanon.
Hizbollah, whose attacks helped to end Israel's 22-year occupation of southern Lebanon in 2000, was the only Lebanese militia to retain its arms after the 1975-1990 civil war.
The group has argued in the past against joining the Lebanese army, saying that would undermine its effectiveness as a guerrilla force and increase the risk that the army become embroiled in confrontations with Israel.
Backed by Syria and Iran, the Shia Muslim group has pledged to keep those arms as a deterrent against Israel and to liberate the Shebaa Farms, a disputed strip on the border between Lebanon, Israel and Syria's Israeli-occupied Golan Heights.