"Thick and black clouds began to blanket Dumai and Pekanbaru limiting visibility especially in the morning," Slamet Riyadi of the provincial BMKG said here on Sunday.
Slamet said on Sunday visibility is limited to one kilometer in Dumai and 2.1 kilometers in Pekanbaru as against normally more than 5 kilometers in the morning.
Hot spots have been detected in various areas in Riau over the past week disrupting flight schedule.
Pelita Air Service aircraft from Jakarta with destination Dumai was forced to diverted routes on Sunday as black clouds in the sky over the Pinang Kampai airport of Dumai.
The ATR 72-500 aircraft finally landed at the international airport of Sultan Syarif Kasim (SSK) II of Pekanbaru.
"The aircraft was forced to land in Pekanbaru, as it was not smokes block view over the Dumai airport," SSK Airport Duty Officer Bambang Setiawan said.
Flights were still normal from and to International Airport of SKK II as the smoke was not as thick as over Dumai airport.
"Visibility in the morning has been improved reaching 5,000 meters on Sunday from 1,800 meters once in the past week," he said.
Last Thursday, the Police Criminal Investigation Agency (Bareskrim) said it had detained 85 forest fire individual suspects in Riau this year.
"In addition, nine companies are being investigated for alleged involvement in forest fires," Bareskrim chief Insp. Gen. Ari Dono Sukmanto said here .
The suspects are facing a jail term of 15 years and a fine of Rp5 billion if found guilty, the police general said.
Last year, 15 plantation and forestry companies in the list of forest fire suspects were released by police drawing protest from environmental activists.
Police said the case against the 15 companies was dropped as there was not enough evidence found against them.