NATO air strikes have forced Libyan government troops to withdraw from one of their positions in the besieged city of Misrata.
However, a rebel spokesman said they have resumed bombardment of the city's port area using Grad missiles and mortars.
Forces loyal to Muammar Gaddafi also fired the Russian-made Grad, a multiple-volley system which rights groups say is highly inaccurate, into the centre of the rebel-held town of Zintan.
'There was intense bombardment this morning. Around 15 Grad rockets landed in the town centre, two of them landed where I'm standing now,' the spokesman, called Abdulrahman, said by telephone from Zintan.
'Five houses were destroyed. Nobody was killed, luckily, but some children were slightly wounded,' he said.
A UN human rights group has arrived in Libya to investigate accusations that pro-Gaddafi forces have violated human rights and attacked civilians.
Libya says security forces were forced to act against armed gangs and al Qaeda sympathisers trying to seize control of the country.
Misrata has become the focus of Gaddafi's drive to break a rebellion against his four-decade rule.
However, neither the army nor rebels backed by NATO air strikes have achieved a decisive victory in weeks of fighting that have destroyed large areas of the town.
UN investigators have arrived in the capital Tripoli and to meet Libyan government officials.
'We have a number of questions dealing with indiscriminate bombing of civilian areas, civilian casualties, torture and the use of mercenaries and other questions,' said Cherif Bassiouni, a member of the UN commission.
Meanwhile, British Defence Secretary Liam Fox said British support given to Libyan rebels, including body armour and military advisers, was not the first step towards arming them.
'We have been very clear that this is mentoring, not training,' Mr Fox told the parliamentary defence committee.
'We believe this is vital to their stated role and their ability to help protect the civilian population better. So it is not a first step, nor is it intended to be.'
