Tightly packed dormitories housing thousands of foreign workers have emerged as one of Singapore’s biggest challenges in its fight to contain the spread of the coronavirus.

Of 142 new infections reported in the city state on Wednesday, at least 40 of those were linked to clusters at foreign worker dormitories that house mainly low-wage workers in construction and other sectors. Those groups accounted for 253 infections across nine facilities, more than 15 percent of the country’s 1,623 cases at the time, according to Ministry of Health data.

Authorities have moved swiftly to isolate the clusters. Two dormitories that together house almost 20,000 people were on Sunday designated by the Ministry of Manpower as "isolation areas” after new, linked virus cases emerged, while a third dormitory was gazetted the following day. Residents were ordered to stay in their shared rooms for two weeks, but would still receive wages as well as deliveries of food and other essentials.