A US State Department spokesman, Nicholas Burns, said Washington is asking officials in Niger and Nigeria for details of the flights.
"We will be very disappointed with both of those governments if in fact they aided and abetted the Libyan dictator to fly in violation of the UN sanctions to both of those countries," Mr Burns said.
Afterward, he said, the US will go to the UN Security Council and the UN Sanctions Committee to seek condemnation of Libya.
The Security Council banned flights to and from Libya in 1992 to force the surrender of two Libyans wanted in connection with the bombing of Pan Am Flight 103 over Lockerbie which killed 270 people in 1988. The sanctions effectively prohibit any flight by Libyan aircraft outside the country.
Mr Burns said he was confident that Mr Gaddafi's travels will undercut countries which advocate a lifting of the sanctions.
Mr Gaddafi was greeted in Nigeria by General Sani Abacha, the country's military leader, and by thousands of school children.
n Lagos (Reuters) - Lawyers for Nigerian dissidents charged with treason over recent bomb blasts asked a court to strike out the case.
The 12 dissidents, including the exiled Nobel laureate Wole Soyinka, face the death penalty if convicted. Gani Fawehinmi, leading other lawyers, told the magistrate her court was incompetent to hear such a serious charge.Reuse content