Illegal forest clearing fires in Indonesia’s Sumatra Island are sending haze across the Malacca Strait to neighboring Malaysia and Singapore, causing the worst air pollution since 2006, officials said on Thursday.

Despite pledge among governments to deter fires, the haze prompted Malaysia to alert vessels in the Malacca Strait of poor visibility as short as 2 nautical miles and shut many schools.

Singapore, covered in thick smoke this week, saw its air pollution index hit the highest level since 2006 on Wednesday. The port and international airport are still functioning as normal.