A day of fierce fighting in Libya has seen rebels trying to hold back an intensified counter-offensive by forces loyal to Col Muammar Gaddafi.
Rebels said they repelled government forces from Zawiya, west of Tripoli, after intense fighting.
Further east, government troops fought their way into Misrata using tanks and artillery, before being forced back.
Helicopter gunships strafed Bin Jawad, prompting a retreat by rebels who had captured the coastal town on Saturday.
Tripoli has been Col Gaddafi's main stronghold as he attempts to reassert control over the country from rebels, who have taken much of the east of the country as well as some towns closer to Tripoli, in the west.
Meanwhile, the UN says Col Gaddafi's government has agreed to allow a "humanitarian assessment" team to visit Tripoli. The UN has appointed former Jordanian foreign minister Abdelilah al-Khatib as its special envoy to Libya.
'Bombs and bullets'
The UN has also demanded immediate access to Misrata to help the injured.
Some of Sunday's heaviest fighting was reported in the city, 200km (125 miles) east of Tripoli, where a local doctor told the BBC the situation became "very bad" after pro-Gaddafi forces with tanks and armoured cars went into the city centre and opened fire.
At the scene
For the first time, Col Gaddafi's forces are standing their ground and the rebels have suddenly found they've got a fight on their hands.
Planes and at least one helicopter have been flying over Ras Lanuf, spying out its defences, and the further the rebels get to Col Gaddafi's tribal stronghold of Sirte along the coast, the more sophisticated is the equipment used by the pro-Gaddafi forces.
We were driving close to Bin Jiwad where the fighting is concentrated when two big mortar bombs landed quite nearby. The rebels don't seem so enthusiastic and excitable any more. Their mood is grimmer and more determined now.
Col Gaddafi's television service in Tripoli has been claiming that his troops have recaptured Ras Lanuf, the strategic oil terminal, but for now it's still firmly in the hands of the rebels. But there is a great deal of nervousness among the defenders at present, and they're firing their guns defiantly into the air from time to time.
He said they shot at people whether they were armed or not, and Reuters quoted a witness as saying at least 18 people were killed before the pro-Gaddafi forces were pushed out of the city five hours later.
A resident of Misrata, Mohamed Benrasali, told the BBC there were joyous scenes there as the Gaddafi forces were turned back.
He said one government tank had been blown up and 16 Gaddafi soldiers killed. Other soldiers had been captured and would be interrogated on Monday.
He vowed "to fight to the last man, woman and bullet" if the Gaddafi forces returned.
With a 300,000-strong population, Misrata is the largest town controlled by rebels outside their stronghold in the eastern part of the country
That stronghold is focused in Benghazi, where they have set up a Transitional National Council that has called on the international community to recognise it as Libya's sole government.
Meanwhile, troops backed by helicopter gunships attacked the major oil town of Ras Lanuf which was taken by rebel forces on Saturday. It is 160km east of Col Gaddafi's stronghold of Sirte.
Rebels said their forces withdrew from Bin Jawad - about 50km north-west of Ras Lanuf - after coming under attack when they advanced.
"We got thrown by bombs and by snipers from the side roads that we can't see," said one rebel, Jamal Al Karrar, of the attack. "All I saw is bombs and bullets and we were trying to escape... It was really scary."
As well as facing stronger resistance, rebels are also running short of fuel, which is increasing their anxiety, correspondents say.
In Zawiya, rebel spokesman Youssef Shagan told Reuters: "This morning, there was a new attack, bigger than yesterday. There were one and a half hours of fighting... Two people were killed from our side and many more injured. We are still in full control of the square."
Humanitarian appeal
Sunday's fierce fighting in Misrata sparked UN calls for immediate access to the "injured and dying" as local residents urged more action from the international community in the form of enforcing a no-fly zone over Libya.
"Humanitarian organisations need urgent access now," said UN emergency relief coordinator Valerie Amos.
"I call on the authorities to provide access without delay to allow aid workers to help save lives."
The UN estimates that more than 1,000 people have died in nearly three weeks of unrest in Libya, which follows public protests in neighbouring Tunisia and Egypt that saw their longtime authoritarian leaders overthrown.
An estimated 200,000 people - mostly foreign workers - have fled the country, creating a humanitarian crisis along Libya's border with Tunisia.
The UN Security Council approved sanctions last week imposing asset freezes and travel bans on Col Gaddafi and his family and aides.
The resolution also referred Col Gaddafi and his inner circle to the International Criminal Court for investigation of crimes against humanity.
In other developments:
- An eight-strong British diplomatic team who were detained near the rebel stronghold of Benghazi were freed. The team, thought to include six SAS members, was found carrying guns, explosives and passports from four countries. It had reportedly been trying to put UK diplomats in touch with the rebel leadership
- French Foreign Minister Alain Juppe said any international military intervention in Libya would have "absolutely negative" effects. But, speaking on a visit to neighbouring Egypt, Mr Juppe said Western nations had to be prepared to react should the violence become bloodier, and should plan for a possible no-fly zone.
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