President Taylor is accused of involvement in three civil wars
Liberian President Charles Taylor has been told to arrest two Sierra Leoneans indicted for war crimes or face prosecution.
Both Sam Bockarie, alias "Mosquito" and Johnny Paul Koroma are in Liberia,
Alan White, chief investigator for the United Nations-backed Special Court for Sierra Leone, told the BBC's Network Africa.
Mr Bockarie was a senior commander of the rebel Revolutionary United Front which committed widespread atrocities during Sierra Leone's brutal 10-year civil war.
Johnny Paul Koroma seized power from the elected leader Ahmad Tejan Kabbah in 1997 and later formed an alliance with the RUF.
Earlier this week, Mr Bockarie was accused of killing a rebel commander in neighbouring Ivory Coast.
The UN prosecutor said that Mr Koroma had been in Liberia for up to a month, while Mr Bockarie crossed the border after Ivory Coast rebels said they wanted him for the killing of Felix Doh.
We have the authority to indict and arrest a head of state for war crime violations
Special Court for Sierra Leone
"The president of Liberia is put on notice that he has two indicted war criminals on his territory," Mr White said.
"If he does not turn those individuals over to the Special Court, he could be subject to prosecution for aiding and abetting known war criminals."
Mr Koroma fled Sierra Leone earlier this year after the authorities tried to arrest him following an attack on a military barracks.
He was elected to parliament last year after receiving a large number of votes from the military.
President Taylor is under UN sanctions for allegedly backing the RUF in their rebellion which ended early last year.
Civilians have paid the price of West Africa's rebellions
He has also been accused of setting up the two rebel groups which operate in western Ivory Coast.
He denies any involvement with them, although his ministers accept that some Liberian mercenaries may be fighting in Ivory Coast.
Mr Taylor came to power in 1997 elections following an eight-year civil war in Liberia.
The UN prosecutor insisted he had the right to pursue the president, but would have to seek permission from the Security Council.
"We have the authority to indict and arrest a head of state for war crime violations," Mr White said.