Six teenagers from a Burundi robotics team have been reported missing after an international competition in Washington and two of them were seen entering Canada, police said on Thursday.

The four males and two females were last seen late on Tuesday afternoon when the robotics contest ended at the FIRST Global Challenge, police said. Authorities issued missing persons photographs of the six on Wednesday.

Two of the Burundians - Audrey Mwamikazi, 17, and 16-year-old Don Ingabire - were spotted crossing the United States border into Canada, District of Columbia police spokeswoman Margarita Mikhaylova said.

"We don't have any indication of foul play and we're continuing to investigate this case," she said. Police said they did not have information about how they were spotted or the nature of the border crossing.

Canada's Border Services Agency said it could neither confirm nor deny that the pair entered Canada.

The international competition drew attention after U.S. President Donald Trump intervened to allow an all-girl robotics team from Afghanistan whose members had been denied U.S. visas to compete.

Burundi has been in the grip of a crisis since President Pierre Nkurunziza's controversial decision in April 2015 to run for a third term in office. He won elections in July that year which were boycotted by the opposition.

Between 500 and 2,000 people have been killed in clashes in the east African country, according to U.N. and NGO sources. More than 400,000 people have fled and dozens of opposition activists have been forced into exile.

The Burundi Embassy in Washington said by email that it did not know about the robotics contest or if a Burundian team was attending.

Competition organizer FIRST Global said in a statement that its president, Joe Sestak, made the first call to police about the missing competitors. The non-profit group learned on Tuesday night that the Burundi team's adult mentor had been unable to find them, it said.

"Security of the students is of paramount importance to FIRST Global," the statement said. It added that FIRST Global had provided safe transport to housing at Trinity Washington University after each day's competition.

Students were always supposed to be under the supervision of their mentor, and had been advised not to leave the university without their mentor, the organizers said.

The other missing Burundians were named as Nice Munezero, 17; Kevin Sabumukiza, 17; Richard Irakoze, 18; and Aristide Irambona, 18. Police said the students had one-year visas.