Washington denies it has plans to offer a $2m bounty for the capture of Liberia's exiled former leader, Charles Taylor.
Liberia's former leader lives in exile in Nigeria
A bill approving an $87bn aid package for Iraq and Afghanistan included a reward for "an indictee of the Special Court for Sierra Leone".
Nigeria said the offer, assumed for Mr Taylor's detainment, verged on state-sponsored terrorism.
The US says the money could be "an additional tool" if the need arises.
"We strongly oppose any violent or other illegal actions against
Nigerian authorities aimed at obtaining custody of Charles Taylor," US State Department spokeswoman Susan Pittman said.
"Apprehension of indictees should be conducted by appropriate authorities."
Mr Taylor went into exile in Nigeria as part of a plan to end Liberia's civil war.
SIERRA LEONE SPECIAL COURT
Established by UN and Sierra Leone
Try those most to blame for war crimes
Mandate till 2005
Local and international prosecutors, judges
Funded by UK, US and others
Ms Pittman said the US had supported the warlord leaving Liberia
because it believed it would save lives.
Many assumed that the Americans would leave the matter there.
Nigeria expressed shock at the reward which it said would encourage lawless and illegal behaviour.
It said it would resist any attempt to capture Mr Taylor, adding that Liberia's ex-leader was under the protection of the Nigerian Government.
Mr Taylor's presence on Nigerian soil was the result of a plan agreed by African nations to resolve the conflict in Liberia, he added.
Security has been increased around Mr Taylor's compound in Calabar, in the far south-east of the country, following reports of the alleged US bounty.
The ex-leader has been there since August when he arrived with around 100 other people, although many have now left. According to a Liberian official close to Mr Taylor, this is partly for security reasons and partly because they found life there rather dull.
Mr Taylor was indicted by the war crimes court in Sierra Leone while he was still Liberia's president.
It is seeking to try him on charges that he armed and trained Sierra Leone's rebels who waged a campaign of rape and dismemberment during the country's civil war.
As a former warlord, he was also involved in the bloodshed in his own country.
Ms Pittman said Washington regarded Nigeria as a good place from which Mr Taylor could "address the indictment".