Libyan government forces are advancing towards the oil port of Ras Lanuf, checking the rebels' westward progress, BBC correspondents say.
Bin Jawad, 50km (30 miles) from Ras Lanuf, has now fallen to forces loyal to Libyan leader Col Muammar Gaddafi.
The UN has named a former Jordanian foreign minister as its envoy to Libya, where the anti-Gaddafi revolt is now well into its third week.
Almost 200,000 people have now fled the violence in Libya, the UN also says.
It is launching an appeal for $162m (£99m) to help 600,000 people within Libya who are expected to need humanitarian aid, in addition to a projected total of 400,000 leaving the country in the short term.
Migrants fleeing
Pro-Gaddafi forces launched an air strike on Ras Lanuf on Monday, news agencies reported.
Mokhtar Dobrug, a rebel fighter who witnessed the air strike, told Reuters: "There was an aircraft; it fired two rockets. There were no deaths."
Meanwhile, pro-Gaddafi forces launched a renewed tank and artillery attack on Zawiya, a rebel-held town 50km west of the capital, Reuters reported an exiled Libyan opposition group as saying.
Events in Libya were "absolutely outrageous", Nato Secretary General Anders Fogh Rasmussen told the BBC.
"These systematic attacks against the civilian population may, as stated by the UN Security Council, amount to a crime against humanity," he said.
However, he said Nato had no plans to intervene, and any operational role would be pursuant to a UN mandate.
Rebels have been trying to fight off a counter-offensive by Gaddafi forces, which have been attacking both near Tripoli and in the east after recent rebel gains.
The UN's latest figures show at least 191,748 people have fled the violence in Libya since the fighting began.
Many more are likely to want to leave but have not made it to a border or are constrained from crossing, said the UN Office for the Co-ordination of Humanitarian Affairs (Ocha).
Meanwhile, Col Gaddafi has warned that Libya plays a vital role in restraining illegal immigration to Europe from sub-Saharan Africa.
"There are millions of blacks who could come to the Mediterranean to cross to France and Italy, and Libya plays a role in security in the Mediterranean," he said in an interview with the France 24 television channel.
'Civilian targets'
UN Secretary General Ban Ki-moon has appointed Jordan's former foreign minister, Abdelilah Al-Khatib, as his special envoy.
A statement from Mr Ban's office said he noted "that civilians are bearing the brunt of the violence, and calls for an immediate halt to the government's disproportionate use of force and indiscriminate attacks on civilian targets".
Mr Ban also said Libyan Foreign Minister Moussa Kusa had agreed to accept the immediate dispatch of a humanitarian assessment team to the capital.
UN relief co-ordinator Valerie Amos said that after heavy fighting in Misrata, 200km (125 miles) east of Tripoli, "people are injured and dying and need help immediately".
A local doctor told the BBC that 21 dead and more than 100 wounded had been brought to his hospital, which he said was also targeted by government troops.
He said the fighting went on for at least six hours.
With a population of 300,000, Misrata is the largest town controlled by rebels outside their stronghold in the eastern part of the country.
Residents have called for the international community to establish a no-fly zone over Libya to prevent Col Gaddafi's air force from attacking.
