President George W Bush has said the US will work to ensure those behind the killing of former Lebanese PM Rafik Hariri are brought to justice.
Many Lebanese still mourn Rafik Hariri
He was speaking after talks at the White House with Lebanon's Prime Minister, Fouad Siniora.
Mr Hariri's assassination in February 2005, was blamed on Lebanon's then political master, Syria.
Damascus denies the accusation, but had to withdraw its troops from Lebanon not long after the assassination.
The Lebanese government is still unable to extend its authority over all the country, says the BBC's Kim Ghattas.
The Lebanese central state has rarely been strong, but Mr Siniora wants to change that now, says our correspondent.
"For the past - over 16 or 18 months - Lebanon has been undergoing major changes," Mr Siniora told reporters after the White House talks.
"Lebanon has really been committing itself that we want the change to happen, in a democratic and a peaceful manner," he said.
"But in the same time to really stay the course - that we're there to meet the expectations of the people, to have a united, liberal, free country and at the same time a prosperous economy."
Lebanon has been under international pressure to disarm all militias in the country.
Bush says he wants Lebanon to succeed as a free country
That means Hezbollah, the anti-Israeli Shia guerrilla movement, which is also represented in the government, and a number of radical Palestinian factions. All these groups are allied to Syria, says our correspondent.
Hezbollah maintains it needs to hold on to its weapons because Israel still occupies a small piece of Lebanese land, known as the Shebaa Farms.
However, the UN has ruled that the Farms are actually Syrian territory, occupied by Israel.
Mr Siniora wants to convince President Bush to put pressure on Israel to withdraw from the Farms regardless of whom they belong to, our correspondent says.
This would undermine Hezbollah's excuse to hold on to its weapons, and allow the Lebanese government to extend its control all over Lebanon.
It is unclear how Mr Bush or Israel will respond to this proposal, says our correspondent.
Mr Siniora, who is from the anti-Syrian coalition, has already been harshly criticised by Hezbollah for his move.