Forces loyal to Libyan leader Muammar Gaddafi are using human shields in the war-torn town of Misrata, Nato says.
The coalition says Col Gaddafi's troops are using civilian vehicles and hiding their heavy armour in civilian areas.
Nato spokeswoman Oana Lungescu told the BBC the besieged western town "remains our number one priority... but [Gaddafi's troops] are using human shields to protect themselves."
Meanwhile, the US has repeated its call for Col Gaddafi to give up power.
Earlier a leading rebel urged Nato must do more to help Misrata.
Gen Abdul Fattah Younis of the anti-Gaddafi forces complained that bureaucracy was causing Nato to take hours to respond to calls for air strikes.
"[Nato] is letting the people of Misrata die every day," he said.
The UN humanitarian chief expressed serious concern, saying the situation in Misrata was "critical".
"We need a temporary cessation of hostilities in the area so that people can get themselves and their families out of harm's way," UN Under-Secretary General for Humanitarian Affairs and Emergency Relief Co-ordinator Valerie Amos said.
Meanwhile, a former US Republican congressman is now in Tripoli to hold talks with Col Gaddafi.
Curt Weldon said his aim was to persuade Col Gaddafi to "step aside".
Mr Weldon said he was visiting the Libyan capital in a personal capacity at the invitation of Col Gaddafi's chief of staff and that the administration of Barack Obama was aware of the trip.
Earlier on Wednesday, Libyan state TV said Col Gaddafi had sent a letter to US President Barack Obama after what he termed the US "withdrawal from the Crusader, colonialist and hostile alliance against Libya".
The White House later confirmed it received the latter, without giving further details.
In the three-page letter Col Gaddafi implores Mr Obama to stop the Nato-led air campaign, which he calls an "unjust war against a small people of a developing country," according to the Associated Press, which says it has obtained the letter.
Deflecting a question about the letter at a Washington news conference, US Secretary of State Hillary Clinton said: "Mr Gaddafi knows what he must do. There needs to be a ceasefire, his forces need to withdraw from the cities they've taken at great cost.
"There needs to be a decision made about his departure from power and his departure from Libya. The sooner that occurs and the bloodshed ends the better it will be for everyone."
The fighting is reported to be taking a heavy toll on civilians trapped in Misrata amid shelling, sniper fire and chronic shortages of basic supplies.
Nato says it has intensified its air attacks on Libya, with an increase of almost 50% in three days. It says it has destroyed one-third of Col Gaddafi's military capacity.
"We've hit defence systems, armoured vehicles, tanks - including around Misrata," Ms Lungescu told the BBC.
Hundreds of Nato sorties have been flown in the past few days, she said, and heavy armour has also been hit around Brega and Ras Lanuf in eastern Libya.
"Nato must remain vigilant to avoid civilian casualties," she said, explaining that the pro-Gaddafi forces had changed their tactics, using lighter forces similar to the rebels in Misrata.
The BBC's Tim Willcox in rebel-held Benghazi says there is a lot of anger in the city about the civilian suffering in Misrata.
Some wounded civilians evacuated from Misrata by boat described horrendous scenes of shelling and hand-to-hand fighting, he says.
A Nato-led coalition mandated by the UN to protect civilians is enforcing a no-fly zone and attacking ground targets. It accuses Gaddafi forces of sheltering weapons in civilian areas.
In Washington the White House confirmed on Wednesday that it had received a letter reportedly from Col Gaddafi, but a spokesman declined to comment on the context of the letter.
This week the US handed over command of the Libya mission to its European Nato allies.
Rebel forces in the east of the country have retreated after heavy bombardment from Col Gaddafi's army, the BBC's Wyre Davies says from the front line, near the town of Ajdabiya.
Heavy and accurate shelling from Gaddafi troops has pushed the disorganised and poorly-equipped rebel army back to the fringes of the town, our correspondent adds.