Rebels have pleaded for more help as Muammar Gaddafi's ground forces recaptured a strategic oil town and moved within striking distance of another major eastern city.
Gaddafi's advance nearly reversed the gains rebels made since international air strikes began.
A US official said government forces were making themselves harder to target by using civilian "battle wagons" with makeshift armaments instead of tanks.
Western powers kept up the pressure to force Gaddafi out with new air strikes in other parts of Libya, hints that they may arm the opposition and intense negotiations behind the scenes to find a country to give haven to Libya's leader of more than 40 years.
Meanwhile, an American official and former US intelligence officer said CIA operatives were sent to Libya this month after the agency's station in the capital was forced to close. CIA officers also assisted in rescuing one of two crew members of an F-15E Strike Eagle that crashed, they said.
Even as it advanced militarily, Gaddafi's regime suffered a blow to its inner circle with the apparent defection of foreign minister Musa Kusa, who flew from Tunisia to Farnborough Airport, Hampshire, and announced he was resigning from his post, the British government said.
Moussa Ibrahim, a Libyan government spokesman in Tripoli, denied the foreign minister had defected, saying he was in London on a "diplomatic mission".
Gaddafi's justice and interior ministers resigned shortly after the uprising began last month, but Kusa would be the first high-profile resignation since the international air campaign began.
Air strikes have neutralised Gaddafi's air force and pounded his army, but his ground forces remain far better armed, trained and organised than the opposition.
The shift in momentum back to the government's side is hardening a US view that the poorly-equipped opposition is probably incapable of prevailing without decisive Western intervention - either an all-out US-led military assault on regime forces or a decision to arm the rebels.