Speaking at a news conference on Thursday, Lee Hsien Loong said the haze that began afflicting Singapore on Monday could persist until the end of the dry season on Indonesia's Sumatra Island in September or October.
Singapore's air pollution standards index soared to a record high of 371 on Thursday afternoon, exceeding the previous peak set in 1997.
Some residents wore face masks as they walked to and from work, but many convenience stores sold out of the items, leaving other people to cover their mouths with handkerchiefs or tissues.
The smell of burned wood was so strong that it even permeated the city's underground train system. Many Singaporeans chose to stay at home and use social media to air their complaints about Indonesia's role in the smog.
Singapore's National Environment Agency chief Andrew Tan met with Indonesian officials in Jakarta to discuss the situation. He called on Indonesia to take "decisive action" against companies suspected of burning forests on Sumatra as a cheap way of clearing land.
In separate remarks to reporters, Indonesian Minister for People's Welfare Agung Laksono criticized Singapore's reaction to the haze, accusing the city-state of acting like a "child."
Indonesia has defended its response, saying it is investigating companies suspected of setting the fires and trying to educate plantation owners about alternatives to traditional burning methods.