(CNN) Four armed men kidnapped an American tourist in Uganda Tuesday and have demanded $500,000 as ransom, the Ugandan Tourism Board and Ugandan police force have said.

The American citizen -- a woman -- and a Ugandan driver were kidnapped at gunpoint at Queen Elizabeth National Park in southwest Uganda, while on a game drive on Tuesday evening, a statement released by the agencies said.

"The kidnappers, using the victim's phone have demanded 500,000 USD," police said, adding "we strongly believe this ransom is the reason behind the kidnap."

Police have dispatched an "elite squad" from the Tourism Police to the park to actively pursue the gunmen and have closed all exit areas on the border between Uganda and the Democratic Republic of Congo.

"We strongly believe the perpetrators and victims could still be trapped within our search area," the police statement added.

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