The US has pledged $5m towards the UN-backed tribunal set up to try suspects in the 2005 murder of Lebanese ex-Prime Minister Rafik Hariri.
Hariri's killing sparked protests that pressured Syria to leave Lebanon
The US ambassador to the UN said the money reflected Washington's commitment to "the process of accountability for political murders" in Lebanon.
The court is to be based in the Netherlands. No suspects have so far been named in Mr Hariri's killing.
An initial UN inquiry suggested Syria played a role - a claim it denies.
The tribunal was set up by a UN Security Council vote in May, and is expected to start work next year.
Its costs over a five-year period have been estimated at $35m. Part of the funding comes from the Lebanese government.
Mr Hariri and 22 other people died in a huge car bombing in Beirut in 2005.
Announcing Washington's contribution to the tribunal, US Ambassador to the UN Zalmay Khalilzad said there should be "no impunity for political assassinations".