Washington - A US air marshal was attacked with a syringe at the airport in Lagos, Nigeria and had been hospitalised, the FBI said on Monday.

It was not known what was in the syringe when it was injected on Sunday into the on-duty marshal, whose name has not been given and is in hospital in Houston.

The incident raised fears the syringe could have carried some form of the Ebola virus.

The FBI said authorities were taking every precaution, but that initial signs were that the victim was not a danger to others and was not showing any symptoms.

"Out of an abundance of caution, the Centres for Disease Control and Prevention conducted an on-scene screening of the victim when United Flight 143 landed in Houston early Monday morning," the statement said.

"The victim did not exhibit any signs of illness during the flight and was transported to a hospital upon landing for further testing. None of the testing conducted has indicated a danger to other passengers."