US officials have cast doubt on reports that Ugandan rebel leader Joseph Kony is negotiating his surrender in the Central African Republic (CAR).

A US State Department official told the BBC that some rebels had been in contact but Kony was not among them.

However, CAR officials later insisted that Kony, who is wanted for alleged war crimes, has been in talks with their government.

The US has offered up to $5m (£3.3m) for leads resulting in his arrest.

Kony founded the Lord's Resistance Army (LRA) in northern Uganda more than 20 years ago, and his fighters are notorious for abducting children to serve as sex slaves and child soldiers.

The US official also noted that Kony had previously used "any and every pretext to rest, regroup, and rearm, ultimately returning to kidnapping, killing, displacing and otherwise abusing civilian populations".

The LRA was forced out of Uganda in 2005 and since then has wreaked havoc in CAR, South Sudan and the Democratic Republic of Congo.

Kony was on the verge of signing a peace deal in 2008 but insisted that the International Criminal Court (ICC) first drop its arrest warrant, which it refused to do.

On Wednesday, a CAR government spokesman told the BBC that Kony was in the country but wanted his security to be guaranteed before giving himself up.

According to AFP news agency, CAR leader Michel Djotodia, said: "Joseph Kony wants to come out of the bush. We are negotiating with him."

A US State Department official said Washington was aware that CAR authorities had "been in contact for several months with a small LRA group in CAR that has expressed interest in surrendering".

But later on Thursday, a spokesman for Mr Djotodia told the BBC's Newshour radio programme that the president had had a phone conversation with Kony, who was in the remote Central African town of Nzoka "with nearly 7,000 people, with his whole group, women and children".

BBC Africa security correspondent Moses Rono says this is not the first time that a breakthrough has been reported with the LRA.

But he says that Kony is now a shadow of his former self, reportedly in fading health, having lost many fighters and operating in a volatile region, with armed gangs keen to earn the $5m reward.

The African Union's special envoy on the LRA, Francisco Madeira, told the UN Security Council on Wednesday he had seen reports that Kony was suffering from a "serious, uncharacterised illness".

In April the Ugandan army suspended a search for Kony in CAR, blaming "hostility" from the government formed when Mr Djotodia's rebel forces seized power there.

Some 3,000 African troops, backed by 100 US special forces, have been hunting him and his fighters across the region.

Mr Madeira said the military pressure had kept Kony and the LRA "on the run".

Kony claims the LRA's mission is to install a government in Uganda based on the Biblical Ten Commandments.

But he is wanted by the International Criminal Court in The Hague on charges of rape, mutilation and murder of civilians, as well as forcibly recruiting children to serve as soldiers and sex slaves.