Former President Charles Taylor is still trying "to influence events" in Liberia, the United Nations has said.
Taylor is supposed to stay out of Liberian politics
Mr Taylor went into exile in Nigeria in August as part of a peace deal which is due to lead to the creation of a power-sharing government on Tuesday.
"Any interference from Mr Taylor could threaten the carefully constructed peace agreement," the UN Security Council warned.
He has been indicted for war crimes by a UN-backed court in Sierra Leone.
He is alleged to have backed the RUF rebels who killed and mutilated many thousands during that country's brutal 10-year conflict.
Nigeria does not have an extradition agreement with Sierra Leone but offered Mr Taylor exile in the south-eastern city of Calabar on condition that he stayed out of Liberian politics.
But as he left Liberia, he said: "God willing, I will be back."
The UN said he needed to be kept on a tighter rein.
"We think that his activities need to be curbed so that he does not remain in political contact with his former supporters," said US ambassador John Negroponte, the Security Council president for October.
"So I think it's very important that he observes the terms of his having left Liberia and that he respects the commitment that he undertook not to pursue political activities from outside the country," he said.
Mr Taylor's ally Moses Blah is due to hand over power to neutral businessman Gyude Bryant as the head of an administration comprising two rebels groups, Taylor loyalists and civilian opposition parties on 14 October.
All sides must keep their weapons out of Monrovia
The world's biggest peacekeeping operation is being sent to Liberia, but they have not yet ventured outside the relative calm of the capital, Monrovia, which has been declared a "weapons-free zone".
Vehicles and commuters are being searched to ensure that no arms are taken into Monrovia.
More Bangladeshi troops are expected to arrived in Monrovia later on Friday, after being switched from similar UN peacekeeping duties in Sierra Leone.
Some 800 Bangladeshis are reinforcing the 3,500 West African troops already in place.
The UN force, Unmil, is due to reach its full strength of 15,000 by next March.