UN head Kofi Annan has said the Democratic Republic of Congo elections in June will pose "major logistical challenges, if not nightmares".
The UN mission in DR Congo is the largest in the world
He is visiting DR Congo, where the UN has its largest peacekeeping mission, ahead of the country's first free elections in 45 years.
He also welcomed a planned deployment of European Union troops to act as a rapid reaction force during the polls.
The polls are to end a power-sharing period after a five-year civil war.
The BBC's Adam Mynott in DR Congo's capital, Kinshasa, says the UN secretary-general is in DR Congo to reinforce the UN's commitment to the country's democratic future and to see for himself how plans for the elections are going.
The present UN force of nearly 17,000 troops is attempting to patrol a country the size of western Europe.
After years of war and misrule, there are no road or railway links from one side of DR Congo to the other.
During his visit, Mr Annan is meeting Congolese President Joseph Kabila and others to press them to make the voting as free and fair as possible.
Shortly after his arrival at UN headquarters in Kinshasa, he said the elections presented DR Congo with a great opportunity which should not be missed.
"We are now at a critical stage and come June we will be organising elections - elections in a huge country with little infrastructure," he said.
"This poses major logistical challenges, if not nightmares."
Mr Annan said the planned deployment of up to 1,500 mainly French and German EU troops - to be based in a neighbouring African country - would support the existing UN peacekeeping mission during the elections.
"It's going to be a rapid reaction force to come in as and when required," he told the BBC's Network Africa programme.
Our correspondent says Mr Annan will be told during his visit that for DR Congo to have any hope of a stable future, more peacekeeping forces are needed.
More than 1,000 people are estimated to be dying each day in the country, where fighting - particularly in the east - continues.
Hostilities between rival militias and government forces continue to force hundreds of thousands of displaced people from their homes.