A US woman who was kidnapped with her driver at Uganda's most popular wildlife park failed to take an armed ranger as required by the park's regulations, according to the country's wildlife authority.

Ugandan police said Kimberley Sue Endecott, 35, and Ugandan driver Jean Paul were on a game drive in Queen Elizabeth National Park when four gunmen ambushed their vehicle on Tuesday evening.

An elderly couple also at the scene were not taken and raised the alarm.

Various illegal groups from Somali militant Islamists to Congolese-based rebels sometimes operate in Uganda, but the kidnappers' identity was not known.

"We have armed ranger guides. If you're going out on a drive in the park, you're supposed to have one but these tourists went out on their own without a guard," a spokesman for the state-run Uganda Wildlife Authority (UWA) said.

"From their camp in the park, they just got into a vehicle and went out.

"They should have notified us and informed us that they're going out for a game drive and then we would have availed them a guard but they didn't do this," he said.

Ms Endecott, who lives in California, and the couple entered Uganda on 29 March and flew the next day to the park in the country's southwest, the spokesman added.

There was no immediate comment on the progress of the investigation by Ugandan authorities.