Government soldiers have surrounded and made their first ground assault on Ajdabiya - the last town before the main rebel-held city of Benghazi.
Tanks are also pounding the last rebel-controlled city in the west, Misrata.
Meanwhile, French President Nicolas Sarkozy has urged the UN Security Council to back a draft resolution that would include a military no-fly zone.
In a letter to member states, Mr Sarkozy said Col Gaddafi had continued his “murderous actions against his people” despite a resolution last month, and that they now had to “assume their responsibilities”.