The International Criminal Court (ICC) on Tuesday marked its 20th anniversary urging all nations to help its "vital work" seeking justice for war crime victims, despite its controversial acquittal of a former Congolese militia chief.

"Two decades after the Rome conference, the system of international justice created by the Rome Statute continues to make waves towards building a culture of accountability," insisted chief prosecutor Fatou Bensouda.

Even though the tribunal, based in The Hague, faced many challenges "its work is increasingly shaping norms, casting a deterrent shadow across the globe," she said.

The tribunal's guiding Rome Statute was agreed in July 1998, and it opened its doors in 2002 as a court of last resort, to prosecute those behind the world's worst atrocities in places where national authorities could not or would not step in.

In 16 years, it has sentenced three people, two Congolese militia leaders and a Malian jihadist.

Other cases have collapsed. In some instances wanted suspects remain at large, including Sudanese President Omar al-Bashir, and four trials are currently underway.

Last month, former Congolese vice president Jean-Pierre Bemba, initially sentenced to 18 years in prison for war crimes, was acquitted on appeal in a blow to the prosecution.

The court has been repeatedly criticised, accused of unfairly targeting African nations, even though complex initial probes are also underway in the Palestinian territories, Afghanistan, the Philippines and Ukraine.