Smoke from forest fires in Indonesia has choked neighboring Singapore, pushing air pollution in the densely populated city to soaring levels.

The haze, blown toward the east and northeast from the Indonesian island of Sumatra, first hit Singapore and Malaysia over the weekend, shrouding cities in thick smog and forcing people to stay indoors.

Pollution levels in Singapore skyrocketed Wednesday, passing the 300 mark on the government’s pollutant index, triggering a major health alarm in the city state.

Singapore’s National Environment Agency said on its website its three-hour Pollutant Standards Index peaked at 155 Monday night, the highest reading since September 1997 when levels notched 226.

In Malaysia, the smog has also reached “unhealthy levels” as the country’s Air Pollutant Index rose as high as 161 on Saturday.

Winds will likely blow more smog across the border until Thursday when skylines are expected to finally clear up. Children, the elderly and those with heart and lung diseases were urged to avoid outdoor activities and immediately seek medical treatment should they feel unwell.

Meanwhile, authorities in Singapore said they had expressed their concern with Indonesia and offered assistance to deal with the fires, which sparked outrage from residents and tourists in the city state.

Indonesian Foreign Minister Marty Natalegawa said his country had taken measures against the haze problem and that it plans to use cloud seeding to try and unleash rain on Sumatra, according to AFP.