Authorities say the oil is seeping from three sources, including a vessel that has not yet been identified.

Oil spilling from an unidentified vessel and two “natural” sources in the Gulf of Mexico has spread to seven nature reserves, covering an area of more than 600km (373 miles), Mexican authorities said on Thursday.

The findings follow weeks of controversy in Mexico over how the spill, which was first reported in early March off the coasts of Veracruz and Tabasco states, has been handled.

Although authorities ruled out “severe environmental damage”, Oceana, an international organisation focused on ocean conservation, said this week that reports from communities in the area revealed the spill has killed sea turtles, a manatee, and various fish species. About 17 reefs have been reported damaged, the organisation said.

Satellite imagery and physical inspections in the affected areas pointed to three sources of the oil spill, Navy Secretary Admiral Raymundo Morales told reporters at the Thursday news conference, adding that the spill point remains active.

They included a vessel docked off the coast of the port city of Coatzacoalcos in Veracruz; a geological site 8km (5 miles) from the Coatzacoalcos port and known locally as a “chapopotera”; and another natural leak site in the Bay of Campeche.

‘Greater flow of contaminants’

Morales said the vessel involved has not been identified because 13 ships in the area had not been inspected by early March.

However, the natural seeps in the Bay of Campeche are believed to be one of the main sources, he said.

“These oil seeps have a constant, natural emission; however, there has been a greater flow of contaminants in the last month,” Morales said.

In a separate incident on March 17, residents of the coastal town of Puerto Ceiba said they witnessed an explosion around the Olmeca refinery – owned by state oil company Pemex – after oily water overflowed onto a nearby road. A vehicle passing by exploded as a result, killing five people, according to a Pemex statement last week. There is fear that oysters in the connected Mecoacan lagoon – and the fishermen harvesting them – will be affected.

Environmental organisations have meanwhile denounced alleged inaction by state authorities over the ongoing Veracruz and Tasbasco case.

In a statement, Greenpeace Mexico said the Gulf of Mexico was being treated by authorities as a “sacrifice zone for the oil industry” and called on the government to act faster in communicating and minimising the damage.

“Federal authorities have the obligation to provide this information to the public in a timely manner and to supervise, regulate, and repair the impacts on marine and coastal ecosystems, as well as to design and implement effective protocols that guarantee the prevention, response, and mitigation of the social and environmental risks associated with oil activities,” the organisation said.

At least six species, including sea turtles, birds, and fish, have been contaminated so far, Mexican environmental authorities revealed.