Singapore said Saturday that it would pursue local firms found to be involved in starting forest fires in Indonesia, as Greenpeace said the blazes were on palm oil plantations owned by Indonesian, Malaysian and Singaporean companies.
Smog has engulfed the city-state with fluctuating levels rising to a record high yesterday, although they had dropped to "moderate" by Saturday afternoon.
Foreign Minister K Shanmugam told a press conference Saturday that Singapore would investigate possible legal action against domestic companies responsible for the fires.
"I have asked the attorney-general to consider what is it that we can do in Singapore if such companies can be proved to have contributed in some way (to the fires) ... We will do everything we can do," he told a press conference.
"We will offer no succour or refuge if the actions of the companies have indeed been illegal in Indonesia and impacted on Singapore," Shanmugam said.
He pressed Indonesia to provide evidence.
"We would have to depend on Indonesia to give us the evidence ... Indonesian investigation authorities need to be on the ground, I cannot send my police officers in there to investigate," he said.
Environmental group Greenpeace said in a statement Saturday: "NASA hotspot data in (Indonesia's) Sumatra over the past 10 days (11-21 June) has revealed hundreds of fire hotspots in palm oil concessions that are owned by Indonesian, Malaysian and Singaporean companies."
Shanmugam told the press conference that he would raise the regional smog problem at next week's Association of Southeast Asian Nations (ASEAN) meeting.
He added that the city-state would pursue the matter at other forums if the ASEAN foreign ministers' meeting in Brunei next week produced no "concrete results."
In a separate interview with local media, Shanmugam, who is also Singapore's law minister, said Singapore would "take all steps, even if it means that our neighbours are upset."
Indonesia last week sought to shift some of the blame for the raging forest fires on Malaysian and Singaporean palm oil companies that had invested in Indonesia.
On Friday, Indonesia's environment minister said it was investigating eight firms for causing the fires but did not name them.
A senior presidential aide, Kuntoro Mangkusubroto, said Friday that the fires happened in concession areas belonging to two paper producers.
Singapore's smog index reached an all-time record level of 401 on Friday.
It hovered over the "hazardous" 300 level on Saturday before dropping to a "moderate" level of 85 in the afternoon.
Government guidelines advise the public to "avoid all unnecessary outdoor activity" at levels above 300.
The haze crisis has had a dramatic impact on life in Singapore, with its residents scaling back their activities in a bid to protect themselves.
But residents rejoiced late Saturday after seeing clear blue skies for the first time in almost a week. Shoppers were out in force along posh Orchard Road.
Southeast Asia's previous major haze crisis in 1997-1998 caused widespread health problems and cost the regional economy billions of dollars as a result of business and air transport disruptions that lasted for weeks.