Kampala, Uganda -- Ugandan security forces scoured dense bush on Thursday in the hunt for a U.S. tourist and her safari guide who were kidnapped by gunmen in a national park. "The operation to rescue the tourist is still ongoing," Uganda's tourism minister, Godfrey Kiwanda, told AFP on Thursday.

Four kidnappers stopped a group of tourists at gunpoint around dusk on Tuesday as they drove through the Queen Elizabeth National Park on safari to see wild animals. Police identified the American as a 35-year-old Kimberly Sue Endicott and said the kidnappers had used her mobile telephone to demand a ransom of $500,000 for the release of the pair. The driver is a 48-year-old safari guide with years of experience.

Police spokeswoman Polly Namaye said all efforts were being made, adding that Uganda's top-ranking police officer, Inspector General Martin Okoth Ochola, had flown to the area to lead operations.

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"The operation is continuing," she told AFP.

The gunmen dragged the pair from their safari vehicle, but left behind two other tourists, whom police described as an "elderly couple." That pair managed to raise the alarm from the lodge where they were staying.

Editor's note: Bashir Hangi, a spokesman for Uganda's state-run Uganda Wildlife Authority (UWA), told the Reuters news agency earlier on Thursday that the small group including Endicott had set out for their tour without an armed guard, which he said was required by the park's rules, but CBS News spoke to multiple tour organizers in Uganda on Thursday who all flatly rejected that claim.

Michael Rourke, of the Wild Frontiers company that organized the safari Endicott was on, told CBS News that Hangi's claim was "factually untrue - there is no rule saying you must have an armed ranger" on tours in Uganda.

Rourke and other organizers told CBS News that safari groups are not even permitted to travel with armed guards in Uganda.

Armed anti-poaching forces, along with Ugandan law enforcement and military, are the only armed personnel permitted to patrol the area, and multiple sources told CBS News that tourists are unlikely to ever see such forces on a tour.

Popular but risky area

The U.S. embassy in Kampala said it was aware of the kidnapping, warning its citizens to "exercise caution when travelling to this area due to ongoing security activity."

Soldiers have fanned out along the porous border with the Democratic Republic of Congo (DRC), with security forces insisting they believe the pair remain in the country.

The Ugandan police's tourist protection force has also deployed a special response unit working alongside soldiers and wildlife rangers.

Queen Elizabeth National Park, one of the East African nation's most popular wildlife reserves, runs along the frontier with conflict-wracked regions of DRC. It borders on the famous Virunga national park, the oldest in Africa.

Numerous militia groups and armed gangs roam eastern DRC. Virunga suspended all tourism activities last year after a ranger was killed and two British tourists kidnapped.

The Britons and their driver were freed two days after the attack. The park reopened in February.

The Ugandan park straddles the equator, covering 764 square miles in the country's southwest region.

Tourism is a key industry for Uganda, as a major earner of foreign currency. Hundreds of thousands of tourists visit each year.

Correction: This story has been updated to correct the spelling of Kimberly Sue Endicott's name.