At least 37 people have died after inhaling toxic fumes while allegedly stealing petrol from a pipeline in Nigeria's Rivers State, an environmental group in the oil-rich region says.

Many are still missing after more than 100 people descended on Okari Jetty in Okrika in small boats to collect the refined fuel under cover of darkness, according to the Youths and Environmental Advocacy Centre (YEAC).

The police told the BBC an investigation was ongoing to determine the figures of those affected after people reportedly collapsed - with local reports saying some fell into the Bonny River while others were taken to hospital.

Theft from pipelines has been a problem for decades in Nigeria's oil hub.

Rivers is one of several states that make up the southern Niger Delta region, a vast area of swamps and wetlands with huge reserves of oil and gas along Nigeria's coast.

For years, criminal groups have tapped pipelines and siphon crude oil or other petroleum products into boats and storage tanks, to be sold on the black market or taken to illegal refineries.

The YEAC said its network of volunteers and human rights defenders had reported that people in Okrika had died after inhaling what it locally known as "Indorama fuel", which is a form of refined petrol known as PMS (Premium Motor Spirit).

Nigeria's Security and Civil Defence Corps (NSCDC) has urged the public and media to rely on verified information as they work with other security agencies to establish what happened in the early hours of Thursday morning.

It condemned illegal tapping and siphoning, warning that such activities put lives, the environment and the economy at risk.

Similarly, many campaigners have also blamed the practice for contributing to environmental damage.

The government has stepped up operations against oil theft in recent years.

On Thursday, the director of navel information, Capt Abiodun Folorunsho, warned thieves there was "no hiding place" from the navy, which patrols the waterways and coastal areas of the Niger Delta.

"Their days are numbered, and if we catch you, of course, we'll ensure that you face the law," he said in an interview with Nigerian broadcaster Channels TV.

While the Nigerian navy has increased maritime security operations, the military and other security agencies have carried out operations against oil thieves, who often hid out in the area's dense mangrove forests.

Nigeria is trying to increase oil production after years of problems caused by theft, insecurity and underinvestment.

The country aims to raise production to about three million barrels per day by 2030 and is relying on better security and reforms to attract new investment.