Image copyright AFP Image caption Firemen have been putting out fires in Riau province on Sumatra island, as well as areas in south Kalimantan island

Indonesia has arrested executives from seven companies accused of being behind illegal fires causing air pollution in the region.

Correspondents say such arrests are rare, but could result in fines and prison sentences of up to 15 years.

Haze caused by forest fires is an annual problem but this year has been one of the worst in Indonesia.

Indonesia has declared a state of emergency in Riau province, and nearby countries are also affected.

Malaysia and Singapore have seen deteriorating air quality from the haze, which stretches hundreds of kilometres across South East Asia.

Read more: Why South East Asia's haze is a hot issue

Image copyright AFP Image caption Many in the region. including Malaysia, have been wearing face masks as the air quality deteriorates

The fires are said to be caused by corporations as well as small-scale farmers who use the illegal slash-and-burn method to clear vegetation for palm oil, pulp and paper plantations.

The problem has accelerated in recent years as more land has been cleared for expanding plantations for the lucrative trade.

Under Indonesian law, executives of companies which are accused of committing "environmental mismanagement" can be criminally prosecuted.

Indonesia has for many years promised to step up enforcement of laws on illegal fires.

But our correspondents say there have been few arrests, and none of those held responsible so far have been from large companies. The arrest of senior executives is thus a significant move.

Indonesia's police chief Gen Badrodin Haiti said on Wednesday night that the seven companies were based on either Sumatra or Kalimantan on Indonesian Borneo, where most of the fires have taken place. He did not say how many executives were arrested.

Those arrested face charges of illegal burning of vegetation to clear land, which has a maximum penalty of 10 years in jail and a 10bn rupiah (£447,000; $692,000) fine, and illegal burning of forests, which has a maximum penalty of 10 years in jail and a 1.5bn rupiah fine.

Image copyright AFP Image caption Indonesia has sent in the army to help out with firefighting efforts

Gen Badrodin said another 20 companies, and a total of 140 individuals, were also being investigated.

Indonesia's President Joko Widodo has vowed to hold "disobedient" plantation companies responsible for illegally clearing land by burning, and for the first time the government has threatened to revoke the licenses of firms found guilty of environmental crimes.

Gen Badrodin has also said police investigations could target high-level executives and even shareholders.

But analysts and environmentalists say they have heard similar promises before and are waiting to see if these tough words result in prosecutions, say our correspondents.

Image copyright AFP Image caption Organisers of Singapore's Formula One grand prix say the event will take place as scheduled this weekend

Indonesia is facing pressure at home and abroad to quickly solve the issue. The haze has led to school closures in Malaysia and sparked regional authorities to issue health alerts.

The organisers of the Formula 1 Grand Prix in Singapore, a highlight on the city's tourism calendar, have announced that the night races this weekend will continue as planned despite uncertainty over the air quality.