An American woman and her driver have been kidnapped at Uganda's most popular wildlife park by gunmen demanding a ransom of $500,000, police said on Wednesday.

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, a police statement said.

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