Ecuador's election authority has rejected a petition for a referendum on oil exploration and development in Yasuni, a unique nature preserve in the Amazon rain forest.

Yasunidos, the environmentalist group that filed the petition, said it had met the law's requirements for a referendum by gathering nearly 728,000 signatures, more than five percent of the electorate.

But the National Electoral Council invalidated half the signatures, citing unspecified irregularities.

"The petition does not have the number of signatures necessary to approve the consultation," the council said in a statement.

The election authority added it would send its report to the country's constitutional court, noting that Thursday's decision was subject to appeal.

Yasunidos said the signatures were invalidated in an untransparent manner.

The Ecuadoran Congress voted last year in favor of drilling for oil in the national park, which is estimated to hold 20 percent of the country's 920 million barrels of crude oil reserves.

Home to two indigenous tribes that have resisted contact with the outside world, the park -- designated a UNESCO Biosphere Reserve -- covers an expanse of more than 9,800 square kilometers (6,090 miles) between two rivers and consists mainly of rain forest.

President Rafael Correa supports going ahead with the plans, after the failure of an international appeal for funds to avoid it.

The project, however, has sparked opposition from several environmental groups, as well as associations of indigenous people worried about the impact of pollution on the Yasuni preserve, which is located in eastern Ecuador.