Embattled Libyan leader Muammar Gaddafi's wife and daughter have fled the country to neighbouring Tunisia, United States Secretary of State Hillary Clinton has said.

"The pressure on the Gaddafi regime has increased to the point that Gaddafi's wife and daughter fled across the border into Tunisia in the last two days," she said.

In an interview to CBS news, Clinton said progress is being made in Libya.

"There is an enormous amount of increased messaging going to Gaddafi, not just because of the military strikes but from those who he thought were in his camp or at least wouldn't try to push him to leave," she said.

"At the same time, the Transitional National Council and their military forces are getting better. They started off as being totally unprepared for what they were confronting. So we are making progress. I wish it would go faster, they certainly wish it would go faster, but we are on the right path," Clinton said.

The Secretary of State has welcomed the release of the foreign journalists in Libya, including two US citizens, and asked the Libyan government to release all US nationals who are still under arrest.

"We call upon the Libyan government to immediately release all US citizens and others who are being unjustly detained," Clinton said in a statement.

"Gaddafi and his regime must allow journalists to carry out their work within Libya, free from fear and intimidation and to respect the universal rights of everyone in Libya, including US citizens and others," she said.

"We will continue to work relentlessly to secure the release of all US citizens unjustly detained in Libya and -- with the help of the international community -- hold accountable those responsible for the reprehensible human rights abuses against the Libyan people and others within Libya," Clinton said.