The Libyan government has accused Nato of bombing a residential neighbourhood in the capital and killing civilians. At least four people, including two children, were reported killed.

It was not possible to independently verify the government's account, and Nato said it was investigating. The alliance has repeatedly said it tries to avoid killing civilians.

Shortly after the air strikes before dawn on Sunday, government officials rushed journalists to the destroyed building, which appeared to have been partly under construction. Reporters were escorted back to the site during the day, where children's toys, teacups and dust-covered mattresses could be seen amid the rubble.

A government spokesman, Moussa Ibrahim, said there were no military facilities anywhere near the damaged building. Journalists were shown at least four people said to have been killed in the strike, including the two children.

Salem Ali Garadi, 51, said his brother and sister were among the victims, and claimed five people had been killed.

The deputy foreign minister, Khaled Kaim, said: "There was intentional and deliberate targeting of the civilian houses. This is another sign of the brutality of the west."

Libya's health ministry has said 856 civilians have been killed in Nato air strikes since they began in March. The figure could not be independently confirmed. Previous government tolls from individual strikes have proved to be exaggerated.

Nato wing commander Mike Bracken said the alliance was looking into the latest reports. "Nato confirms that it was operating in Tripoli last night, conducting air strikes against a legitimate military target. Nato deeply regrets any civilian loss of life during this operation and would be very sorry if the review of this incident concluded it to be a Nato weapon," he said.

A Nato mission spokesman in Naples, who spoke on condition of anonymity, said the Gaddafi regime had made false claims in the past about Nato having caused civilian deaths.

The alliance appeared to strike Tripoli again on Sunday afternoon. A number of explosions could be heard in the city, and smoke could be seen rising over the southern part of the capital.

Nato warplanes have stepped up their campaign over the past week, and fighting has intensified between rebels and government troops outside the port city of Misrata, the main rebel stronghold in western Libya. The eastern third of the country is under rebel control from their de facto capital, Benghazi.

On Sunday, Gaddafi's forces unleashed a heavy barrage of Grad rockets and mortars on the rebel frontlines in Dafniya, about 15 miles west of Misrata. Muthana Issa, an official at Misrata's Hikma hospital, said four people had been killed and 16 wounded in the early hours of the bombardment.