Ugandan President Yoweri Museveni has met negotiators from the rebel Lord's Resistance Army for the first time in an effort to revive stalled talks.
Museveni flew into Juba amid tight security
A Ugandan minister said Mr Museveni went to southern Sudan to support and encourage the peace talks there.
But an LRA spokesman said the meeting lasted only five minutes and the Ugandan president had been abusive.
Negotiations began in July to try to end the violent 20-year conflict in northern Uganda.
The LRA spokesman, Godfrey Ayo, told the BBC that Mr Museveni "only called us uninformed Ugandans who have been out of the country for 20 years.
"He also said other things which were all abusive - indicating that he is never interested in peace talks."
Uganda's Deputy Foreign Minister Henry Okello Oryem denied to the BBC that Mr Museveni had been abusive.
"To the contrary, [Mr Museveni] used the opportunity to make it very clear that he had come all this way to support and encourage the peace process."
The meeting was in Juba, southern Sudan, where talks between the two sides have been taking place.
After addressing the two negotiating teams, Mr Museveni went to shake hands with the LRA representatives but was refused.
A planned news conference was cancelled.
The killing of 38 civilians in southern Sudan earlier this week has cast a long shadow over the Juba peace talks, says our correspondent there, Jonah Fisher.
Women and children were among the victims shot in the head before their vehicles were set on fire.
The LRA has denied carrying out the attack, but our correspondent says after a period of calm there is fear the rebels have returned to their brutal ways.
For nearly 20 years the LRA has been attacking civilians and abducting children in northern Uganda and southern Sudan.
Mr Kony's army fought the Ugandan government for almost two decades
Their leader, Joseph Kony, says he wants to rule Uganda according to the Bible's 10 commandments.
LRA fighters were supposed to be gathering at two points in southern Sudan in return for amnesty from the Ugandan government.
But the Ugandan military violated the agreement by moving troops toward the sites. Fearing an attack the rebels withdrew from the assembly camps.
Charges of war crimes from the International Criminal Court for Mr Kony and four other rebel commanders have also been a sticking point.
The rebels want the indictments dropped before agreeing to any peace deal.
Kampala says it will not consider amnesty for the men until a peace agreement is reached.