More than 1,000 workers have tested positive for the coronavirus at a Sri Lanka factory that until August was producing surgical masks for the US, officials said, 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 asymptomatic or lacking display of symptoms associated with Covid-19,” the firm said in a statement.