In the message, the speaker urged Libyans not to surrender to what he called "imperialist" forces," CNN reported Friday.
The message said the Libyan capital was moved from Tripoli to Sirte, Gadhafi's birthplace.
"We are ready for a long, drawn-out war," the man purported to be Gadhafi said, adding rebels "will not be able to fight a long war. They will retreat, day by day."
"We will fight against you wherever you are," the message said. "We will sacrifice our lives so that the sand and stones of Libya will become fire, and fight against you. You will never have peace of mind inside our land."
Meanwhile, the U.N. said Friday a team arrived in Tripoli to respond to severe shortages of water, food and fuel, CNN reported.
"The humanitarian situation remains fragile," Panos Moumtzis, the U.N. humanitarian coordinator for Libya, said in a statement.
"It is critical to ensure an immediate and effective U.N. presence on the ground to help identify and assist vulnerable people who have been particularly affected by the conflict and the disruption of services."
Along with addressing food and water shortages, the team will try to come up with ways to protect civilians, officials said.
UNICEF will work with the National Transitional Council, the interim government, in hopes of finding and distributing bottled water to 500,000 people, the statement said. The World Food Program has sent enough food to help 35,000 people for a month and enough gas to "cover immediate life-saving humanitarian needs" for a month, the statement said.
The U.N. estimates the water shortage affects 60 percent of Tripoli, with 1.6 million people, and aid workers are trying to determine how to distribute loads of bottled water ships have started bringing in.
Gadhafi's whereabouts were not revealed in the audio message broadcast on Syria's al-Rai television. Gadhafi has not been seen publicly since Tripoli fell.
The message warned Gadhafi loyalists in Sirte and other strongholds were armed and ready to fight to the death.
It came several hours after conflicting reports surfaced that rebel leaders extended, by one week, a Saturday deadline for Gadhafi followers in Sirte and other Gadhafi strongholds to surrender, CNN reported.
The National Transitional Council military commander in Tripoli said the extension was an attempt to prevent more bloodshed.
However, another military official told CNN transitional leaders were still discussing options and that "as of now [Thursday], the deadline is this Saturday."
The Nigerian government called on the National Transitional Council to investigate reports of abuse of civilians stranded in the country when the uprising began months ago, the Daily Trust reported.
The Foreign Affairs Ministry said Thursday most of the abusive behavior was directed toward migrant workers and other Africans who couldn't flee. The government said reports indicated civilians were killed, raped and extorted.
"This development is a deviation from the overall expressed desire of the NTC, the African Union and indeed the United Nations for the restoration of democracy and good governance in Libya," the ministry said in a statement. "[The reports] run contrary to Nigeria's call for the leadership of the NTC to be magnanimous in victory and can only stand in the way of peace-building, early reconciliation and reconstruction in Libya."
The BBC reported Mohi Alghazali, a Libyan living in Aberdeen, Scotland, said five relatives -- an aunt, uncle and three of their children -- had died in an attack during the hunt for Gadhafi in Bani Walid. The deaths were said to have occurred this week.
A NATO spokesman told the BBC the organization is in contact with Alghazali and "taking his concerns very seriously." The spokesman called the Bani Walid situation "extremely complex and dynamic."