* Libyans say British air strike hits oil field
* Gaddafi forces launch fresh assault on Misrata
* France sees risk of NATO getting "bogged down"
* Rebels regain ground near oil port Brega
(Libya accuses NATO of hitting oilfield, changes dateline)
By Maria Golovnina
TRIPOLI, April 7 (Reuters) - Libya said a U.N.-mandated British air strike had hit its major Sarir oilfield killing three guards and damaging a pipeline connecting the field to a Mediterranean port.
"British warplanes have attacked, have carried out an air strike against the Sarir oilfield which killed three oilfield guards and other employees at the field were also injured," Deputy Foreign Minister Khaled Kaim told reporters.
There was no immediate official comment from Britain's Ministry of Defence on Kaim's comments about the field.
Earlier, Muammar Gaddafi's forces unleashed mortar rounds, tank fire and artillery shells on the western city of Misrata on as a French minister said NATO air strikes in Libya risked getting "bogged down".
Misrata, Libya's third city, rose up with other towns against Muammar Gaddafi's rule in mid-February, and it is now under attack by government troops after a violent crackdown put an end to most protests elsewhere in the west of the country.
Rebels are angry at what they perceive to be a scaling back of operations since NATO took over an air campaign, following an early onslaught led by the United States, France and Britain that at one stage tilted the war in the rebels' favour.
French Foreign Minister Alain Juppe said Gaddafi forces were making it harder for alliance pilots to distinguish them from civilians by hunkering down in populated areas. "The situation is unclear. There is a risk of getting bogged down," he said.
Juppe told France Info radio he would address the issue of tactics shortly with the head of NATO, adding Misrata's ordeal "cannot go on". NATO has accused Gaddafi of using human shields to make targeting harder for its warplanes.
Civil war in the vast North African desert oil producer ignited in February when Gaddafi tried to crush pro-democracy rallies against his 41-year rule inspired by uprisings that have toppled or endangered other autocrats across the Arab world.
Stalemate on the battlefield in eastern Libya, defections from Gaddafi's coterie and the plight of civilians ensnared in fighting or running out of food and fuel has spurred a flurry of diplomacy in pursuit of a peaceful solution.
But such efforts have made little headway, with the rebels adamant that Gaddafi step down while the government, aware of the limitations of Western intervention, has offered concessions hinting at democratisation but insists he stay in power.
In a blow to rebel finances, Gaddafi forces halted production at rebel-held oilfields in eastern Libya, a rebel spokesman said on Wednesday. Rebels want to resume exports to raise revenue for their uprising.
Oilfields in Misla and the Waha area were hit by Gaddafi's artillery on Tuesday and Wednesday, spokesman Hafiz Ghoga told reporters in the rebel stronghold of Benghazi.
The Liberian-registered tanker Equator sailed from Marsa el Hariga, near Tobruk, on Wednesday apparently with the first cargo of crude sold by rebels since the uprising began in February, shipping sources said.
U.N.-mandated air strikes have so far failed to halt attacks by the Libyan army in besieged Misrata, where residents said snipers on rooftops and tanks firing on populated areas of the city have had a devastating effect.
"Gaddafi forces have changed tactics and are using human shields in urban areas, including in Misrata," Britain's Foreign Office said on Wednesday.
The head of Libya's rebel army has condemned NATO for its slowness in ordering air strikes to protect civilians, saying the alliance was "letting the people of Misrata die every day".
Juppe said: "We've formally requested that there be no collateral damage for the civilian population ... That obviously makes operations more difficult."
But General Abdel Fattah Younes was adamant that Gaddafi was conducting massacres. "Day by day people are dying. Hundreds of families are being wiped off the face of the earth. Patience has its limits," he said.
Asked whether he found NATO's argument that it is trying to prevent civilian casualties convincing he said:
"No, it's not convincing at all. NATO has other means. I requested there be combat helicopters like Apaches and Tigers. These damage tanks and armoured vehicles with exact precision without harming civilians."
Libyan officials deny attacking civilians in Misrata, saying they are fighting armed gangs linked to al Qaeda. Accounts from Misrata cannot be independently verified as Libyan authorities are not allowing journalists to report freely from there.
Rebel criticism has put the Western military alliance on the defensive, particularly over Misrata. Spokeswoman Carmen Romero said that "the pace of our operations continues unabated. The ambition and the position of our strikes has not changed".
NATO air strikes are targeting Gaddafi's military infrastructure but only to protect civilians, not to provide close air support for rebels, much to their dismay, as part of a no-fly zone mandated by the U.N. Security Council.
Relieving the siege of Misrata was a NATO priority but alliance officials conceded that Gaddafi's army was proving a resourceful and elusive target.
"The situation on the ground is constantly evolving. Gaddafi's forces are changing tactics, using civilian vehicles, hiding tanks in cities such as Misrata and using human shields to hide behind," Romero told reporters in Brussels.
Misrata on Wednesday faced another heavy bombardment.
"There was firing on three fronts today, the port in the east, the centre around Tripoli street and the west of the city. Mortars, tank fire, and artillery were used to shell those areas," rebel Abdelsalam said by telephone.
"NATO needs to either launch a serious operation to take out all the heavy armoured vehicles, including tanks ... If they don't want to do this, they should provide us with weapons to do it ourselves."
Meanwhile, living conditions in Misrata worsened.
"People are panicking, especially women, children and old people. Most people left their homes for safer areas and found refuge with other families," Abdelsalam said, adding:
"No fruit and vegetables have been available in Misrata for over 25 days, bread is also difficult to find. People are scared to go out because of the snipers and the indiscriminate shelling. The upper-hand is still with Gaddafi's forces."
On the eastern battlefield, Libyan rebels regained ground in a new advance on the oil port of Brega on Wednesday but also accused NATO of inaction hindering their quest to oust Gaddafi.
Ill-trained insurgents thrust westwards to the contested port, recovering mostly desert terrain lost in a pell-mell retreat from Gaddafi's superior firepower the day before.
Rebels returning to the tiny outpost of al-Arbaeen, midway between Brega and their frontline town of Ajdabiyah, spoke of rocket duels close to Brega's port as both sides strived to end a ragged stalemate in the civil war.
Rebel Idriss Abdel Karim complained of a lack of NATO support. "(Government forces) are scared of NATO air strikes but NATO doesn't bomb anything in the first place," he said.
"There have been no air strikes. We hear the sound, but they don't bomb anything," said Hossam Ahmed, another rebel.
"What is NATO waiting for? We have cities being destroyed. Ras Lanuf, Bin Jawad, Brega, and Gaddafi is destroying Misrata completely," said Ajdabiyah resident Said Emburak.
Additional reporting by Brian Love and Nick Vinocur in Paris, Mariam Karouny in Beirut, Justyna Pawlak in Brussels, Simon Cameron-Moore in Ankara, Angus Macswan in Benghazi, Tim Castle, Joseph Nasr, Mariam Karouny and Marie-Louise Gumuchian; writing by Mark Heinrich and Peter Millership