Security officials are intensifying their search for an American tourist and her local guide kidnapped Tuesday evening after armed men ambushed them on a game drive in a national park in southwest Uganda.

The pair have been missing since four gunmen held a group of three tourists and their guide at gunpoint, then “grabbed two out of four . . . before disappearing with them,” police said in a statement. The other two tourists escaped unharmed.

The kidnappers have since “used the victim’s phone” to demand $500,000, police said.

A U.S. State Department official told The Washington Post on Thursday that Washington is “aware of reports that an American citizen was kidnapped in the Ishasha area of Queen Elizabeth National Park” and that Ugandan “security forces are responding to the area.”

The State Department has not identified the American woman. Reuters identified her as Kimberley Sue Endecott.

Earlier this week, at an event unrelated to the Uganda incident, Secretary of State Mike Pompeo addressed the families of American hostages and stressed that “any payment to a terrorist or a terrorist regime gives money so that they can seize more of our people.”

“We cannot accept that risk. You wouldn’t ask that of us,” Pompeo said. “Even a small payment to a group in, say, Africa can facilitate the killing or seizure of tens or even hundreds of others, including Americans or foreign nationals in that region.”

Ugandan police said Wednesday that they had dispatched an elite group of police officers, military personnel and wildlife authority officials to assist in the search. The park where the ambush occurred is a popular tourist destination close to the border with Congo.

On Wednesday, police said in a statement that they “strongly believe” the kidnappers and the victims “could still be trapped within our search area, and we are hopeful that our efforts will lead to their successful recovery.”

But on Thursday, CNN quoted a police spokeswoman, Polly Namaye, as saying that “joint security teams have cut off all exit areas on the border between Uganda and [Congo] in search for the victims.”

In May, Ugandan police said that more than 40 kidnappings had been reported in recent months, causing “growing anxiety in our society.” In June, Ugandans took to the streets to protest what they call government inaction in confronting the problem, which disproportionately affects women. After several kidnapping victims were killed, activists left coffins in front of parliament in protest.

Ugandan Ambassador to Washington Mull Katende stressed to The Post on Thursday that it is very “unusual” for a tourist to be kidnapped in Uganda, a peaceful country with a booming tourism industry based largely on its national parks and wildlife. Katende said this incident is “the first time I’m hearing” of a tourist kidnapping.

Bashir Hangi, a spokesman for the Uganda Wildlife Authority, told Reuters that tourists in the park are not supposed to partake in game drives without an armed ranger but that this group “went out on their own without a guard.”

“From their camp in the park, they just got into a vehicle and went out,” he said. “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.”

According to a report from Uganda’s Tourism Ministry, tourism is the country’s “leading foreign exchange earner,” bringing in $1.37 billion in 2016 alone. Tourists flock to the East African nation for game drives in its 10 national parks, birdwatching in protected wildlife sanctuaries, boat trips on the Nile River and hiking in the snow-capped Rwenzori Mountains.