SINGAPORE - Some Sengkang residents have complained of a strong acrid smell and smoke that permeated several areas in Singapore on Monday (Sept 25).

Several readers wrote in to The Straits Times from as early as 4pm on Monday afternoon to say they smelled a "burning", "chemical", or "petrol" smell.

Readers from various areas including Sengkang, Seletar, Yishun, and in other areas like Ang Mo Kio and Bishan, also wrote in to ask about the smell, which one described as "choking".

The National Environment Agency said in a statement on its Facebook page at 10.34pm that they received feedback from the public about a gas smell in Sengkang and Punggol around 5pm.

"NEA officers were deployed to those areas immediately to investigate the smell feedback," it said. "Air quality levels during the period were found to be well within safety limits."

NEA added that winds converged over the northern half of Singapore from about 3pm, which may have led to smells accumulating in the northern area of Singapore.

"Winds are expected to turn to blow from the south or south-east later tonight, which will help to disperse any smells in the area," said NEA.

It added that it will continue to monitor the situation closely.

The Straits Times understands that the fumes are believed to be from Johor.

The Singapore Civil Defence Force said in a Facebook post at 7.42pm that it has been receiving calls on the smell of gas or burning in the north-eastern part of Singapore.

"SCDF deployed its resources to investigate. There has been no confirmed incident," it said, adding that it was closely monitoring the situation along with NEA.

It added at 9.55pm that its monitoring teams have not detected the presence of toxic industrial chemicals in the air.

About 20 calls had been made to the Singapore Civil Defence Force since late afternoon, ST understands.

Ms Tina Lim, 49, who works in a bank, said there was a "very strong burnt smell" in the air when she reached her home in Sengkang at about 5.30pm. "It didn't smell like haze. It's like something is burning."

The smell could be detected even in Ang Mo Kio. Ms Vivian Tan, a 35-year-old administration executive, told ST that she detected a "weird smell" like diesel at Ang Mo Kio Avenue 3 since 6.40pm.

Ms Lelavathi Annamali, who lives in Ang Mo Kio Avenue 8, told ST that there was "a very strong kerosene smell" around 6pm.

She went out and returned home at about 7.10pm and the inside of her home was very smoky, she said.

The smoke stung her eyes and affected her throat.

"The smell is very bad inside the house and is affecting the clothes too," said the 68-year-old retiree.

Bishan resident Jacky Kung told ST that there was a petrol-like smell around 4pm.

"The smell was petrol-like, quite pungent. I could feel my throat drying out and it felt a bit uncomfortable and choking," he said.

He went outside to see what had happened when the smell got stronger, and the skies seemed "a bit hazy", he said.

"I was wondering if it's a local thing that affected only my neighbourhood, but I went up to the highest floor of my block and it looked like it expanded across the horizon," he said.

The 18-year-old student said the smell had died down somewhat at about 8pm and was most intense between 4pm and 5pm.

He closed all the windows and that helped, he said.

Mr Osman Ahmad, a 59-year-old security guard at Seletar Country Club, told ST he noticed the smell of burning chemicals when he left his home at Pasir Panjang around 2pm.

When he stopped to transfer buses at Seletar he noticed the stench was even stronger and that people at the bus stop were covering their mouths and noses with tissue and handkerchiefs.

"You could see a layer of smoke on the highway. It looked like a morning fog, which was beautiful but very smelly," he said.

He added that the smoke disappeared around 7.30pm and it was the first time in his three years at the job that the smell had been so bad.

"Usually it smells like the normal haze, but today you could tell that it was a chemical smell."

sengkang haze right now. you can smell burnt rubber or petrol. pic.twitter.com/Ojd5FzVgmc — rafnexcorp (@rafnexcorp) September 25, 2017

Is it haze? It’s so foggy at Sengkang west and there’s some kind of petroleum smell? pic.twitter.com/ZF7FAxS3Eb — lq (@aixinjueluooo) September 25, 2017

Others likened the smell to burning plastic bags.

The smell appeared to have spread to other parts of Singapore later on Monday night, with readers saying they picked it up in Toa Payoh and even Clementi.

NEA posted an update on the haze situation on its Facebook page at 6.38pm.

It said that the 1-hr PM2.5 concentration readings are expected to stay in Band I (Normal) for the next 24 hours, and the 24-hour PSI is forecast to be in the moderate range.

Several people left comments on the post asking about the burning smell, with some asking if it is toxic.

NEA later shared SCDF's post about monitoring the situation.