Secretary of State Antony Blinken on Wednesday said four Nigerian nationals were killed in an attack on two US government vehicles in Nigeria.
Mr Blinken pledged to support local law-enforcement efforts to hold the perpetrators accountable.
He said unknown assailants on Tuesday in Anambra state attacked the two US vehicles carrying nine Nigerians: five employees of the US mission to Nigeria and four Nigerian police officers.
In his statement, Mr Blinken condemned the attack, but said it did not appear to be directed at the US mission in Nigeria.
“We condemn in the strongest terms this attack," he said. "We will work closely with our Nigerian law-enforcement colleagues in seeking to bring those responsible to justice.
“We do not yet know the motive for the attack but we have no indications at this time that it was targeted against our mission,” he said.
Mr Blinken said the convoy was travelling before a planned visit by US personnel to a US-funded flood-response project in Anambra state.
“The United States has no greater priority than the safety and security of our personnel,” he said.
“We express our heartfelt condolences to the families of those killed in the attack and pledge to do everything possible to safely recover those who remain missing.”