British oil company Soco International paid off an army officer accused of silencing critics of exploration in Democratic Republic of Congo's Virunga National Park, an NGO alleged on Wednesday.

Global Witness published scans of four cheques totalling $15,600 (14,000 euros) allegedly issued by Soco's Congolese subsidiary to the major posted to the park, a UNESCO World Heritage Site that is home to many of the world's critically endangered mountain gorillas.

The British NGO also published what it said was a receipt for the cheques signed by the officer on May 15, 2014, along with another dated April 30, 2014, confirming he had received $26,650 (24,000 euros).

"These documents show that, despite Soco's repeated denials, the company has paid tens of thousands of dollars to an army officer accused of bribing and intimidating those trying to stop oil exploration in one of Africa's natural treasures," Nathaniel Dyer of Global Witness said.

"These payments may only be the tip of the iceberg," he said, calling on Britain's Serious Fraud Office and the US Department of Justice to launch a full investigation into Soco's practices in Virunga.

Soco has previously denied breaching British bribery laws and condemned the use of violence and intimidation, and last year appointed law firm Clifford Chance to look into the allegations surrounding the Virunga project.

In a statement Wednesday, it said this review had concluded that allegations of bribery were "substantially inaccurate", and that there was no evidence the firm or its staff promoted or supported any intimidation of opponents.

However, there were "non-material instances where those with whom the company worked made payments in breach of group policy. These are subject to remedial advice".

Clifford Chance found that Soco had paid "various governmental authorities (eg military escorts and park rangers) for the performance of legal and legitimate activities", the oil company said.

"They found no evidence that any payments were made to secure any unentitled advantage or to curry special treatment," it added.

The oil firm said that it had engaged Clifford Chance to further advise on "improvements in employee/contractor policies and processes", to protect it from similar allegations in future.

Global Witness says local and international NGOs have accused the army officer they name, and the troops under him, of beating, detaining and even killing opponents of Soco's work.

In 2010, the Congolese government granted French oil giant Total and British group Soco permits to explore concessions in Virunga park, but the resulting outrage caused it to suspend them a year later.

Total and Soco subsequently agreed not to enter into the limits of the park, although the latter said it intends to finish a seismic study requested by the government.

Once this is completed in the middle of 2015, Soco says it will have no further involvement in the concession.