Nato chief Anders Fogh Rasmussen has arrived in Tripoli ahead of the official end of the alliance's mission in Libya, BBC reported.
He said Nato could help Libya's new rulers with security and the transition to democracy if requested, AFP news agency said.
Nato forces began operations in March against Col Muammar Gaddafi's forces as they moved to crush an uprising.
They were acting under a UN Security Council mandate to protect civilians.
The Nato mission formally comes to an end at one minute to midnight Libyan time (21:59 GMT) on Monday.
Mr Rasmussen said he would be holding talks with National Transitional Council (NTC) leaders, including its chairman Mustafa Abdel Jalil, "about their expectations as regards Libya's future and in particular their roadmap for transition to democracy".
He would also raise "their expectations as regards possible Nato assistance in the future," AFP said.
Nato could help with "defence and security reform", he said, but it was time for the United Nations to take the lead in international assistance for Libya.
Earlier, he said Operation Unified Protector was "one of the most successful in Nato history''.
The first missions were flown on the evening of 19 March, as Col Gaddafi's forces approached the rebel-held city of Benghazi.
With the help of America's massive military machine, Nato managed to sustain the Libya operation.
Overall, its warplanes flew more than 26,000 sorties, including nearly 10,000 strike missions. More than 1,000 tanks, vehicles and guns were destroyed, along with Col Gaddafi's command and control network.
Mr Rasmussen said Nato's military forces had prevented a massacre and saved countless lives.
"We created the conditions for the people of Libya to determine their own future," he said.
Despite the expected formal announcement that Nato's mission is over, Western powers are likely to be involved in Libya for some considerable time, says the BBC's Jonathan Beale.
The Security Council decided to end its role, despite a call by the NTC for Nato to continue its military action.
The Libyan envoy to the UN had said the NTC needed more time to assess its security needs. But diplomats said that the mandate to protect civilians had been accomplished, and any further security assistance would have to be negotiated separately.
A small team of military advisers remains on the ground to aid the NTC. US and British experts are also trying to ensure that the surfeit of weapons in the country do not end up in the wrong hands.
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