image sourceGetty Imagesimage captionMs Harris flew to Hanoi from Singapore on Tuesday
US Vice-President Kamala Harris's flight to Vietnam was delayed by several hours due to an "anomalous health incident" with similarities to so-called Havana syndrome, reports say.
The mysterious syndrome first affected people at the US embassy in Havana in 2016 and may be caused by directed microwave radiation.
Ms Harris was in Singapore ahead of her visit to Vietnam at the time of the incident in Hanoi.
It is unclear who was affected.
CBS News reported that the incident in the Vietnamese capital is similar to previous incidents of Havana syndrome reported elsewhere.
In a statement, the US State Department said that Ms Harris's departure from Singapore to Hanoi was delayed after reports of a "possible anomalous health incident" there.
The department added that "after careful assessment", Ms Harris and her delegation decided to continue the trip to Hanoi, where she has now arrived.
A senior official quoted by CBS News said that at least one official had to be medically evacuated over the weekend. He added that the incident is not the first case of Havana syndrome reported in Vietnam.
NBC News has reported that two officials were evacuated after "acoustic" incidents were reported in the homes of two US diplomats.
The BBC has contacted the state department to ask for more details.
Since the syndrome was first reported in Cuba in 2016, cases of the condition have been reported elsewhere in the world, including China and, last month, Austria.
Hundreds of US diplomats, spies and other personnel have reportedly fallen ill with symptoms including ear ringing, nausea, and severe headaches.
- Is Russia targeting CIA spies with secret weapons?
A 2019 US academic study found "brain abnormalities" in the diplomats who had fallen ill in Cuba.
A report last year from the US National Academies of Science found the mystery illness was most likely to be caused by directed microwave radiation.
In June, US Secretary of State Antony Blinken announced a review into the causes of the illness.