WASHINGTON (AFP) - The United States (US) offered US$5 million (S$6.2 million) on Wednesday for the capture of Lord's Resistance Army chief Joseph Kony, one of the world's most wanted men, and posted rewards for three other rebel leaders evading trial for war crimes.

The announcement came just as Uganda and Washington said they had been forced to suspend their two-year hunt for Kony in the jungles of the Central African Republic, after rebels seized power in Bangui.

The LRA, a Ugandan rebel group, has waged a brutal insurgency, accused of mutilations and child abductions for two decades across four countries.

Kony - a self-proclaimed prophet who claims his rebels are fighting to establish a government based on the Biblical Ten Commandments - and other LRA leaders face charges of war crimes and crimes against humanity at the International Criminal Court.

Kony's name was added to the State Department's war crimes rewards program along with fellow LRA members Okot Odhiambo and Dominic Ongwen, and Sylvestre Mudacumura from the Democratic Forces for the Liberation of Rwanda (FDLR), in the hopes that the men would be brought to justice.

The LRA "for almost 20 years has tormented and terrorised children across Uganda, the DRC, the Central African Republic and South Sudan. It has to stop", US Secretary of State John Kerry said.

But he admitted Kony and his cronies would "not be easy to find".

"The LRA is broken down into small bands of rebels, scattered throughout dense jungle, hidden by dense canopy, controlling territory through tactics of fear and intimidation," he said in a column in the online Huffington Post.

The LRA was "one of the world's most brutal armed groups", Ambassador for Global Criminal Justice Stephen Rapp told reporters, unveiling the rewards.

"We act today so that there can be justice for the innocent men, women and children, who've been subjected to mass murder, amputation, enslavement and other atrocities," he said.

"Accountability is a key pillar of the United States atrocity prevention initiative."

The United Nations says about 450,000 people have been displaced by LRA attacks in the Democratic Republic of Congo, Central African Republic, Uganda and South Sudan.

Although the number of LRA attacks was down last year, there were some assaults as far west as Bangassou in Central African Republic, where scores of men, women and children were abducted in September.

US President Barack Obama last year renewed a mission by 100 US special forces, first launched in 2011, to help Ugandan troops scour the African jungles for Kony, but it has had no success so far.

Turning to the Democratic Republic of Congo, Rapp said the country has been "plagued by conflict, displacement and insecurity".

"Innocent civilians have suffered continued atrocities at the hands of armed groups such as the FDLR and M23, that support themselves by pillage of the population and exploitation of precious minerals."

Mr Kerry said Mudacumura "has committed and ordered brutal attacks on civilians as the military commander" of the FDLR.

The FDLR is made up of remnants of the radical Hutu regime that carried out the 1994 Rwandan genocide, while M23 is a mainly Congolese Tutsi rebel group UN experts say is backed by Rwanda and Uganda, charges denied by both countries.

The rewards program had "proven to be a valuable tool" in hunting down those wanted for "the worst crimes known to human kind by generating valuable tips", Mr Rapp said.

In the past two years, the program has made 14 payments of an average of US$400,000 per person "with the largest payment being US$2 million", he said.

But Mr Kerry stressed "this is not a dead-or-alive bounty programme".

"Information must lead to the secure arrest, transfer, or conviction of these people in a court of law. We want these men to look into the eyes of their victims and answer for their actions."