Cambodia imposed fresh coronavirus restrictions Thursday for eight provinces bordering Thailand, clamping down on the movement of millions of residents to curb the spread of the highly contagious Delta variant.

In recent weeks, scores of Cambodian migrant workers have returned home from Thailand, which is battling a Delta-fuelled Covid-19 wave.

Cambodian health authorities raised the alarm this week, citing 39 new Delta cases -- 21 in migrant workers returning from Thailand and the rest in contacts.

The clampdown in northwest Cambodia curbs travel to and from Thailand, and between the eight impacted provinces, according to an announcement by Prime Minister Hun Sen on Wednesday.

It will remain in place until August 12, and affects a quarter of Cambodia's population of 16 million.

While the import of goods will continue from Thailand, Cambodian migrant workers wishing to return would have to wait until after the two-week period is over, the announcement said.

The provinces include Banteay Meanchey -- where key border town Poipet is located -- and Siem Reap, home to the famed Angkor Wat temple complex.

Provincial governors will determine which areas are to be designated "red zones", Hun Sen said, a label that effectively bans residents from leaving their homes, gathering or conducting business.

So far, none of the eight provinces has said which areas will have the strictest measures.

Authorities in the capital Phnom Penh also on Thursday imposed a nighttime curfew for two weeks, restricting residents from leaving their homes between 9 am and 3 am.

Cambodia has one of the weakest healthcare infrastructures in Southeast Asia, but it appeared to have largely contained its coronavirus cases last year.

But an outbreak detected in February has steadily driven the caseload up to more than 75,000.