Activists on Tuesday accused Swiss authorities of encouraging impunity, after prosecutors shut a case against a company suspected of laundering gold pillaged by armed groups in Democratic Republic of Congo.

The Swiss attorney general's office decided in March to close a case against Argor-Heraeus, which faced allegations of "complicity in war crimes and pillage" after it refined three tonnes of gold ore pillaged from the conflict-torn country in 2004-2005.

In its ruling, which was quietly made public a month later, the prosecutor's office said there was not enough evidence that the Swiss firm, one of the world's largest processors of precious metals, was aware of the illegal origin of the gold.

The gold, believed to have been illegally mined by a group called the National Integrationist Front (FNI), was sold through a Uganda-based company and on to British Hussar Limited before reaching Argor.

Argor, which claimed ignorance of the gold's origin, welcomed the decision and told AFP it "never doubted the decision since all allegations are without cause".

But Track Impunity Always, or TRIAL, which filed the criminal complaint against the company that spurred the attorney general to launch its probe in November 2013, voiced disbelief at the decision to drop the case.

"It is completely incomprehensible," TRIAL director Philip Grant told AFP, insisting that Argor should have known, or at least suspected, that the precious metal did not come from gold-poor Uganda but had been pillaged in DR Congo.

He pointed out that South Africa's Rand Refinery, which initially refined the gold, had stopped working with Hussar after becoming suspicious of the origin of the metal.

Argor's decision to take over allowed FNI to more easily bring the pillaged gold to market to finance their brutal operations, Grant said, charging the company helped "fuel the conflict".

The FNI was created with Ugandan support and in 2003 took control of a mineral-rich area in the conflict-ravaged northeastern Ituri region of the DR Congo.

In a statement, TRIAL and two other non-governmental organisations warned the Swiss decision "gives free rein to companies who violate their duty of diligence and prefer to remain ignorant of the criminal origin of raw materials."

The decision, they said, "undermines international efforts to eliminate the illegal resource trade that fuels conflicts around the world."

The organisations said the ruling also raised serious questions about Switzerland's role in the precious metals trade.

They questioned "how several tonnes of dirty gold coming from one of the bloodiest and publicised conflicts of our time can reach Switzerland and be refined there without any consequences."