More than 1,000 workers have tested positive for coronavirus at a Sri Lanka factory that until August was producing surgical masks for the US, officials said Wednesday, as they scrambled to find the source of the country's biggest outbreak.

The flurry of cases since the first positive test at the garment factory was reported three days ago pushed the island nation's infections total near 4,500.

Factory owner Brandix, hailed internationally for its sustainable practices, said efforts to contain the explosion of infections at the site -- near the capital, Colombo -- were hampered by a high number of asymptomatic carriers.

"A majority of the positive cases have proven to be an asymptomatic or lacking display of symptoms associated with Covid-19," the firm said in a statement Tuesday.

Sri Lanka's chief epidemiologist, Sudath Samaraweera, called on local communities to cooperate with authorities amid fears the outbreak could spread further.

"We are trying to trace how the virus got in, but we may or may not succeed," Samaraweera told reporters in Colombo.

Until recently, the government had said there was no community spread in the country of 21 million people.

Officials said 1,026 workers who tested positive at the factory -- which employs 1,700 in total -- were in hospital.

The remaining staff members, as well as all the workers' family members, were in the process of being moved to government quarantine facilities.

All of them were expected to be tested for the coronavirus in the next few days.

The region where the factory is located, as well as nearby communities, were placed in an indefinite lockdown.

All public gatherings were banned, schools were closed five days before term break, a domestic cricket tournament was cancelled and the army postponed its 71st-anniversary celebrations scheduled for Saturday.

Brandix, one of Sri Lanka's largest garment exporters, in August delivered 200 million three-ply surgical face masks to a US client, which has not been revealed.

The affected factory was one of those making masks for the order, but has been manufacturing sportswear since August.