Singapore on Tuesday stepped up the pressure on Indonesia to fight forest fires as smog from Sumatra island continued to shroud the densely populated city-state.

Prime Minister Lee Hsien Loong said on his Facebook page that Singapore's foreign and environment ministers had spoken to their Indonesian counterparts to "register our serious concerns and offer our help to fight the fires".

"We are monitoring the situation closely and will stay in close touch with the Indonesian authorities," he said in a posting accompanied by a photograph of the Istana government complex grounds shrouded in haze.

Singapore's Pollutant Standards Index again shot above the "unhealthy" threshold of 100 late Tuesday after easing earlier in the day.

The index had peaked at 155 on Monday night, Singapore's worst outbreak of cross-border air pollution since 2006.

Singapore Foreign Minister K. Shanmugam said he and his Indonesian counterpart Marty Natalegawa "agreed that officials on both sides should meet soon to discuss how best to combat the haze issue".

Vivian Balakrishnan, Singapore's minister for environment and water resources, said of the smog: "This is the worst in seven years -- and has practically become a permanent fixture every year."

He suggested that Indonesia name the companies responsible for the fires.

"We need to exert commercial pressure against companies causing the haze," Balakrishnan said.

On Monday, an Indonesian forestry ministry official, Hadi Daryanto, shifted some of the blame to Malaysia and Singapore, saying their palm oil companies that had invested in Indonesia were also responsible.

"We hope the governments of Malaysia and Singapore will tell their investors to adopt proper measures so we can solve this problem together."

Regional environmentalists said the problem must be tackled by both Singapore and Indonesia.

Indonesia is Southeast Asia's largest nation while tiny Singapore, just minutes away by fast boat from Indonesian territorial waters, is the region's financial hub.

"Singapore is saying that Indonesia needs to enforce the law," Bustar Maitar, head of Indonesia Forest Campaign at campaign group Greenpeace International, told AFP.

"But in actual fact, some palm oil plantations in Indonesia are listed in Singapore and have headquarters in Singapore. A lot of Malaysian plantations are also based here in Indonesia," Maitar said.

"In my perspective, it is not only Indonesia. Singapore should also ask its companies who invest in Indonesia to not use fire, doing the same thing to enforce the law and increase environmental awareness."

Jose Raymond, executive director of the independent Singapore Environment Council, said Singapore and Indonesia should publicly name the plantation owners using slash-and-burn land clearing.

"The public can stand together in letting their voices be heard and this can have a considerable impact on the way these landowners conduct their commercial activities," Raymond told AFP.

He said a review was needed to "rework rural land-use habits and explore other viable and environmentally sustainable forms of economic revenue generation" in Indonesia.

A spokesman for conservation group WWF Indonesia said part of the problem stemmed from "a lack of field officers who can control fires".

"The plantations do slash and burn activities because it is cheaper and easier to do so," he said.

The most severe occurence of the haze problem in Southeast Asia was in 1997-1998, when air transport, tourism and commerce were disrupted, causing billions of dollars in business losses.