At least 10 civilians are reported dead and more than 100 people wounded in the latest fighting in Misrata.
Rebels are battling for control of a major road in their last bastion in western Libya.
The UN’s human rights chief says alleged cluster bomb attacks by the regime could be international crimes.
NATO’s Libyan commander is calling on civilians to keep away from Gaddafi’s forces to help air strikes.
Two western photographers are among those killed. Tim Hetherington, a British-American Oscar-nominated filmmaker was working for the US magazine Vanity Fair.
He and award-winning American photographer Chris Hondros were among a group of journalists caught in a mortar attack.
Reports say they came under fire from government forces. Tripoli says it will investigate.
“Listen, we don’t kill anyone who does not fight us,” said Libyan government spokesman Moussa Ibrahim. “We need to check the circumstance in which that the journalists died. And it is war; of course, people die from our side and from their side. People get caught in the middle. We don’t know the circumstance. We need to check the circumstance. But of course we are very sad for someone died.”
On Twitter recently Tim Hetherington had described “indiscriminate shelling” in Misrata.
Chris Hondros died in hospital hours after the attack, in which two other photographers – a Briton and an American – were injured.