"We state our unequivocal condemnation of this act of terrorism," Secretary of State Condoleezza Rice said in a statement read by spokesman Richard Boucher.
Hariri was killed Monday in a powerful explosion that hit his convoy in Beirut. An obscure Islamist group claimed responsibility.
Rice's statement called for a thorough investigation, adding the United States and the international community will follow the matter closely. Boucher said Washington was consulting with members of the U.N. Security Council "to see what we think the appropriate message from the Security Council might be at this point."
He reiterated U.S. calls for a Syrian withdrawal from Lebanon in line with U.N. Security Council Resolution 1559.
"The United States takes this opportunity to once again call for the ... withdrawal of all Syrian forces," he said.
Syria is the main power broker in the country. Hariri had recently called for its withdrawal.
Boucher said Rice had spoken to U.N. Secretary-General Kofi Annan and will discuss the events with French Foreign Minister Michel Barnier.