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 town's defenders have been given until September 3 to surrender.

However, fugitive ex-leader Col Muammar Gaddafi's spokesman, Moussa Ibrahim, rejected the ultimatum, the Associated Press reports.

"No dignified honourable nation would accept an ultimatum from armed gangs," he said in a telephone call to the AP on the night of August 29.

Mr Ibrahim reiterated Col Gaddafi's offer to send his son Saadi to negotiate with rebels and form a transitional government, the agency said.