Health officials warned Kuwaitis to stay inside and seal doors and windows tightly on Wednesday after toxic gas leaked from an oil well north of Kuwait City.
Residents in areas as far as 100 kilometres (60 miles) away reported smelling the rotten-egg odour of potentially poisonous hydrogen sulphide gas.
Kuwait Oil Co said it had set fire to the well in the huge Rawdatin oilfield close to the border with Iraq, which had sharply reduced the leak.
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KOC chairman Sami al-Rasheed told state television monitoring stations across Kuwait had found hydrogen sulphide concentrations far below danger levels.
The oil and gas leak happened in the afternoon due to a very high pressure of crude but the incident caused no casualties, KOC said earlier.
An industry source said KOC evacuated workers from near the well as a precautionary measure.
Kuwait pumps around three million barrels per day of crude oil whose income makes up more than 94 percent of the Gulf state's revenues.