Libya's interim leadership has rejected the idea of deploying any kind of international military force, the UN envoy to the country has said.
Ian Martin said the UN had considered the deployment of military observers.
Earlier, the chairman of the National Transitional Council (NTC) said the country did not need outside help to maintain security.
The news came as fighters loyal to the council approached the pro-Gaddafi stronghold of Sirte from east and west.
The city's defenders have been given until Saturday to surrender.
However, fugitive leader Col Muammar Gaddafi's spokesman Moussa Ibrahim rejected the ultimatum.
"No dignified honourable nation would accept an ultimatum from armed gangs," he said in a telephone call to AP news agency on Tuesday night.
Mr Ibrahim reiterated Col Gaddafi's offer to send his son Saadi to negotiate with rebels and form a transitional government, the agency said.
Libya's deputy representative to the UN, Ibrahim Dabbashi, told the BBC that the situation in Libya was unique.
"[The UN] put the possibility of deploying peacekeepers on the ground but in fact the Libyan crisis is a special case.
"It is not a civil war, it is not a conflict between two parties, it is the people who are defending themselves against the dictatorship."
However, Mr Martin said the UN did expect to be asked to help establish a police force.
"We don't now expect military observers to be requested," he said after a meeting of the UN Security Council.
"It's very clear that the Libyans want to avoid any kind of military deployment of the UN or others."
Mr Martin added that one of the greatest challenges for the UN would be helping the country prepare for democratic elections.
"Let's remember... there's essentially no living memory of elections, there's no electoral machinery, there's no electoral commission, no history of political parties, no independent civil society, independent media are only beginning to emerge in the east in recent times.
"That's going to be quite a challenge, sort of organisationally, and it's clear that the NTC wish the UN to play a major role in that process."
UN Secretary General Ban Ki-moon has said that growing humanitarian shortages in Libya demand urgent action and appealed to the security council to be "responsive" to requests from the transitional authority for funding.
Though stockpiles of medical supplies and food stashed away by the government were found over the weekend, water supplies are short.
"An estimated 60% of Tripoli's population is without water and sanitation," he said. The EU's humanitarian office says that pro-Gaddafi forces are responsible for cutting supplies.
The EU says it is close to finalising the lifting of sanctions against 28 organisations - including six port authorities and some oil firms.
Officials said the move could come into force within days, and would make it easier for the transitional government to restart trading.
On Tuesday, the UN Security Council allowed the UK to release 1.86bn dinars ($1.55bn; £950m) in frozen assets to buy aid for Libya, but an attempt by France and Germany to release an additional £5.3bn remains blocked.
Diplomats said that Russia was holding up Germany's request to release about £900m in seized assets and France's move to unfreeze about £4bn to buy humanitarian aid, AFP news agency reports.
As anti-Gaddafi fighters converge on his birthplace of Sirte, interim leaders gave the city's defenders an ultimatum, telling them that they had until Saturday to surrender or face military force.
It has also emerged that Col Gaddafi's wife and three of his adult children fled to neighbouring Algeria in the early hours of Monday morning.
Col Gaddafi's whereabouts remain unknown, with suggestions he may be in Sabha, Sirte or Bani Walid. However, the deputy head of the NTC, Ali Tarhouni, said they had a good idea of where he was and were confident that they would catch him.
On Monday, several of his family members - including his daughter Aisha and two of her brothers - fled to neighbouring Algeria. The rebels say they want to extradite them to face trial.