An American tourist and her driver were kidnapped by gunmen in Uganda, local officials said Wednesday.

An elite police unit was deployed to search for the pair after they were ambushed while on safari in Queen Elizabeth National Park on Tuesday night, according to the Ugandan government and police.

Officials said the American woman was aged 35.

The assailants used one of the victims' cellphones to call authorities and demand $500,000 for their release, police said, adding that they "strongly believe this ransom is the reason behind the kidnap."

At least two other people, named by police as Martin and Barbel Julius, were traveling with the group near the border with the Democratic Republic of Congo.

Four kidnappers abducted the American and the driver, taking their keys but leaving the vehicle behind, according to police. The others in the vehicle escaped unharmed and later contacted authorities. The government earlier said that four people had escaped the incident.

Police said they have blocked the nearby border in an attempt to corner the kidnappers.

"We strongly believe the perpetrators and victims could still be trapped within our search area," police said. "We are hopeful that our efforts will lead to their successful recovery."

Queen Elizabeth National Park, Uganda's most popular tourist destination, is home to elephants, hippopotamuses, lions and chimpanzees.