American tourist kidnapped in Uganda's Queen Elizabeth National Park: Officials The American tourist and a Ugandan guide were both abducted.

LONDON -- An American woman was kidnapped on Tuesday evening while visiting Uganda's most popular national park, authorities said.

Four unidentified gunmen allegedly ambushed a group of tourists who were on an evening game drive with a guide in the Queen Elizabeth National Park in southwest Uganda around 6 p.m. local time Tuesday. They held the group at gunpoint before fleeing with their guide and 35-year-old American tourist Kimberly Sue Endicott, according to the Uganda Police Force.

A Canadian couple was left behind without a key to the vehicle. They contacted the camp manager who brought them to safety, officials said.

The tour guide, Congolese national Jean-Paul Mirenge Remezo, works for Wild Frontiers Uganda, a company spokesperson told ABC News. The American tourist, Endicott, was one of the three guests on a safari excursion when she and the guide were both abducted, the spokesperson said.

The kidnappers used one of the victim's mobile phones to demand $500,000 as ransom, according to police.

"We strongly believe this ransom is the reason behind the kidnap," Uganda Police Force deputy spokesperson Polly Namaye said in a statement Wednesday.

A joint operation by police, armed forces and wildlife authorities is underway to locate and rescue them, according to Ugandan government spokesman Ofwono Opondo.

The Queen Elizabeth National Park is located near Uganda's border with the Democratic Republic of the Congo. Authorities have "cut off all exit areas on the border" in the search for the victims, according to Namaye.

"We strongly believe the perpetrators and victims could still be trapped within our search area, and we are hopeful that our efforts will lead to their successful recovery," she said, adding that people shouldn't be discouraged from visiting the national park in the future. "Strengthened safety measures have been put in place for both the local residents and visitors."

A spokesperson for the U.S. Department of State told ABC News, "We are aware of reports of a U.S. citizen kidnapped in Uganda. Security forces are responding to the incident. We take seriously any threats against U.S. citizens abroad. The safety and protection of U.S. citizens overseas are among our top priorities. We have no further information to offer at this time."

U.S. Secretary of State Mike Pompeo, after meeting with the Democratic Republic of the Congo's newly-elected president in Washington, ignored a question from reporters on Wednesday about the kidnapping.

ABC News' Ben Gittleson, Luis Martinez and Cindy Smith contributed to this report.