The US chief of staff for the mission in Libya has insisted there have been no reports of civilian casualties caused by allied action.
Rear Admiral Gerard Hueber's comments come despite claims to the contrary by Muammar Gaddafi's government.
Earlier, British Air Vice Marshal Greg Bagwell said Col Gaddafi's air force "no longer exists as a fighting force".
This has been the fifth consecutive night of air strikes by the US-led international force over the country.
Witnesses said there had been an explosion at a military base in the Tajura region east of Tripoli.
Late on Wednesday, there were reports of government tanks shelling the hospital in the rebel-held western city of Misrata.
Witnesses had earlier said the tanks encircling the city had pulled back from their positions under air assault from international forces.
There were also reports of fierce fighting between rebels and pro-Gaddafi forces in the strategic eastern town of Ajdabiya. Residents fleeing the town described shelling, gunfire and houses on fire.
Rear Admiral Hueber was speaking to reporters by phone from the command ship USS Mount Whitney in the Mediterranean.
"We are putting pressure on Gaddafi's ground forces that are threatening cities," he said. Asked if that meant air strikes, he replied: "Yes."
He continued: "Our mission here is to protect the civilian populace and we choose our targets and plan our actions with that as a top priority."
He added that allied aircraft had flown 175 sorties in the last 24 hours - 113 of them by US aircraft.
His comments came as Nato members debated who should lead the intervention, with the US keen to hand over operational control to Nato.
AVM Bagwell said the allies could now operate "with near impunity" over the skies of Libya.
Speaking during a visit to RAF aircrew based at Gioia del Colle in southern Italy, he said they were now applying unrelenting pressure on the Libyan armed forces.
"We are watching over the innocent people of Libya and ensuring that we protect them from attack," he said. "We have the Libyan ground forces under constant observation and we attack them whenever they threaten civilians or attack population centres."
US Secretary of State Hillary Clinton has again urged Col Gaddafi to step down and leave Libya.
UN Secretary General Ban Ki-moon has urged all sides in Libya to cease hostilities. "All those who violate international humanitarian and human rights law will be held fully accountable," his spokesman Martin Nesirky said.
Meanwhile, ships from Nato nations have started patrolling off the Libyan coast to enforce a UN arms embargo against Col Gaddafi's government.
A spokesman for the Western military alliance, Canadian Brig Gen Pierre St Amand, said six vessels were taking part in the first day of patrols.
They aim to intercept and board ships suspected of ferrying arms to the Libyan government.
"If, after inspection, doubts remain as to the legitimacy of the cargo, the vessel will be diverted to a designated port for further inspection," said Gen St Amand.
Nato members have been holding talks about assuming responsibility for the no-fly zone over Libya, so far without agreement.
Turkey is an integral part of the naval blockade but has expressed concern about the alliance taking over command of the no-fly zone from the US.
US Defence Secretary Robert Gates is in the Egyptian capital Cairo for talks on both Libya and Egypt's hoped-for transition to democracy following the fall of Hosni Mubarak.