According to the Ho Chi Minh City Center for Disease Control, they entered Tay Ninh Province through unmanned trails at 1 a.m. on Wednesday. The trio, a man and two women, took a taxi from Tan Bien Town to HCMC and stayed at a hotel near Tan Son Nhat International Airport.

Health authorities were still trying to identify the hotel as of Thursday morning.

The taxi driver, learning from them they had come from a pandemic-hit area, returned immediately to Tan Bien and contacted health authorities for testing.

The trio then took another taxi to the airport and boarded Vietjet flight VJ134 to Hanoi at 9:10 a.m. on Wednesday.

From Noi Bai airport in Hanoi, the women took a taxi to their hometown in Dinh Hoa, a rural district in the northern province of Thai Nguyen.

At 2 p.m. authorities went to their houses and ordered them to go to a quarantine facility where both tested negative for the new coronavirus.

The man was found in Que Vo District in Bac Ninh Province that evening, also quarantined and again tested negative.

HCMC authorities are trying to trace everyone who had come into contact with the three from the moment they entered the city.

Cambodia has been suffering a spike in infections following its latest wave in late February. Its caseload had jumped to 7,747 with 54 deaths as of Wednesday.

There had been just around 400 infections until last year, but 618 last Sunday alone, 431 on Monday and 303 on Tuesday.

The country’s capital, Phnom Penh, and a satellite district went into lockdown on April 15 and declared some districts as "red zones," banning people from leaving their homes except for medical reasons.

Since last week Vietnam has intensified vigilance along the border.

It has a 1,137-kilometer border with Cambodia running through provinces of An Giang, Dong Thap, Kien Giang, and Long An in the Mekong Delta, Binh Phuoc and Tay Ninh near HCMC and Kon Tum, Gia Lai, Dak Lak, and Dak Nong in the Central Highlands.

In Kien Giang, which shares a land border of 56 kilometers (34.8 miles) and a maritime border with Cambodia, pandemic prevention and control activities have been raised to the highest level.

Everyone entering Vietnam from abroad is quarantined for 14 days and tested at least twice.

For almost a month now Vietnam has not had any community transmission, but the number of imported cases has been increasing gradually, including many people coming from Cambodia.

So far 1,242 of the total of 2,812 cases have been imported.