The RUF often cut off civilians' arms and legs with machetes
Liberia's ex-President Charles Taylor rejected complaints from Sierra Leone rebels about atrocities committed by his fighters, his ex-deputy has said.
The RUF rebels were notorious for mutilating civilians but their leader Foday Sankoh complained to Mr Taylor about Liberian troops, said Moses Blah.
Mr Blah told Mr Taylor's war crimes trial that his former boss responded by threatening to withdraw his troops.
Mr Taylor denies 11 counts of crimes against humanity and war crimes.
Mr Blah, who briefly succeeded Mr Taylor as president, is the most senior figure to testify in The Hague.
He is accused of helping Sierra Leone's Revolutionary United Front (RUF) rebels in their brutal conflict with the government.
Mr Blah told the court that RUF rebel leader Foday Sankoh personally complained to Mr Taylor about the rape, looting and killings committed by Liberians helping the RUF.
"He [Mr Taylor] said this kind of thing must happen when you're fighting war," he said.
The RUF has a reputation for brutality, such as chopping off the arms and legs of civilians, as well as killings and rape.
Mr Blah also said that Mr Taylor mediated in arguments between the rebels.
He said on one occasion a rebel commander had disagreed with the Liberian president's judgement but said he had to accept "the chief's decision".
Mr Blah further said that RUF fighters tried to help Mr Taylor's forces against Liberia's Lurd rebels, who eventually forced him from power in 2003.
Mr Taylor sat taking notes during his former deputy's testimony.
Mr Blah earlier said that the commander of one of Mr Taylor's units in Liberia "had a habit of eating fellow human beings".
He said he saw Nelson Gaye roast a human hand on a fire and eat it with boiled cassava.
Mr Blah said that Mr Gaye's Marine unit was one of Mr Taylor's forces, which he had been told should not be pursued if they committed atrocities.
But Mr Taylor is only on trial for his alleged role in the Sierra Leone conflict, not that in Liberia.
As Mr Blah took the witness stand on Monday, he was careful not to look at the man he served for nearly 20 years.
He gave the court a detailed account of Mr Taylor's bloody rise to power and acknowledged the use of child soldiers.
Mr Blah became vice-president of Liberia in 2000. He became president after Mr Taylor was forced into exile in 2003.
However, he was only Liberian president for two months, until October 2003, when a United Nations-backed transitional government was sworn in.
He came to the Hague reluctantly, but said he intended to tell the truth.
"I'm not going to crucify him - he's my former boss," Mr Blah told the BBC's Focus on Africa programme before travelling to the Hague.
"I have nothing personal against President Taylor - we worked together almost like brothers; we had a revolution going together, so I don't think I'm going to betray him."
In court, Mr Blah provided a detailed insider's account of the early days of the civil war.
He said he was one of the first 20 or so rebel fighters who received training at a military base in Burkina Faso.
Charles Taylor denies war crimes and crimes against humanity
It was not until a larger group was assembled at a military camp in Libya that Mr Blah met Mr Taylor.
They trained alongside rebels from The Gambia, Sierra Leone and the Philippines, all of them supported by the Libyan authorities.
He told the court that when the time came to invade Liberia, three truck loads of weapons were provided by the defence minister of Ivory Coast.
Within days of crossing into Liberia in 1990, Mr Blah says he was briefly detained by child rebel fighters recruited by his own side.
He said they were used because they took orders and were "unreasonable and aggressive".
A lawyer representing Mr Taylor, Terry Munyard, told the BBC his client was not worried about Mr Blah's evidence and if he told the truth there was nothing to fear.
Mr Taylor took up arms in Liberia in 1989, before being elected president in 1997 after a peace deal.
He is accused of sending guns and fighters to Sierra Leone in exchange for diamonds.
During Sierra Leone's decade-long civil war, which officially ended in 2002, tens of thousands of people died and thousands more were mutilated or raped.
Mr Taylor's case has been transferred from Sierra Leone to The Hague for security reasons, although it is still being conducted by the UN-backed Special Court for Sierra Leone.