In September 2009, troops under then junta chief Moussa Dadis Camara opened fire on opposition supporters rallying at a stadium in the West African country's capital Conakry. At least 157 people were killed, while 109 women were raped.

An investigation wound up in 2017, identifying a dozen suspects, including Camara.

But despite repeated promises by Camara's elected successor, President Alpha Conde, the trial has been repeatedly delayed, sparking criticism from rights campaigners.

Guinea's military junta, which toppled Conde last September, has ordered the trial to start before the massacre's 13th anniversary on September 28, although no date has yet been announced.

ICC officials including deputy prosecutor Mame Mandiaye Niang arrived on Monday night.

Niang said the trip was prompted by junta chief Colonel Mamady Doubouya's instruction to his justice minister to launch the trial before the anniversary.

"It was important for us to be here to see the state of trial preparations," Niang, a Senegalese, told reporters.

"Our wish is that this trial takes place so that the ICC is not required to exercise its authority. We expect a fair trial to be held that will respect the rights of the defence and civil plaintiffs," Niang said.

"We are here to help the process, we will do monitoring to see if everything goes well. We will also offer a sort of expertise and experience to ensure the trial's success," he added.

During their visit, the ICC officials will meet with Guinean judicial authorities and victims' groups, a Guinean judicial source said.