Forty civilians have been killed during an air strike on Tripoli, the Vatican's top official in the city has said .

Quoting what he said were reliable reports from residents, Bishop Giovanni Innocenzo Martinelli told the Guardian that 40 people living in the Tajoura district had been killed during a raid "two to three days ago" on a barracks. "There is a barracks there, but it is in the middle of the neighbourhood," he said. "There could have been an arms depot there and if you hit that, you can harm civilians."

Martinelli said he had not seen the site of the bombing, but had been told by a person he trusted. "They said Libyan families had spoken of dead and wounded family members," he said. "There have been many raids here and they are continuing, aimed at all the barracks in the city. "Can you imagine the damage around these sites?"

Martinelli said he had also heard reports of a house collapsing in Buslim, again near a barracks.

"There were certainly people inside, although I do not know how many," he said.

Nato took control of aerial operations over Libya from the US on Thursday, and Nato's senior commander, Canadian Lieutenant General Charles Bouchard, said he would investigate Martinelli's claims, adding that anyone attacking civilians would be "ill-advised" to continue. "I am aware of this news report," he said. "I take every one of those issues seriously, but our mission began … today."

Bouchard said Nato selected targets carefully. "We have very strict rules of engagement provided to us and we are operating within the legal mandate of our United Nations mandate."

Earlier, Martinelli told the Catholic news agency Fides that life in Tripoli was becoming harder every day as the "so-called humanitarian air raids" continued.

" "The fuel shortage has worsened, as evidenced by the queues of cars at petrol stations. On the military level things seem to be at an impasse, because the rebels do not appear to have enough strength to move forward. For this reason I say that the diplomatic solution is the best way to end the bloodshed between the Libyans, giving Gaddafi a dignified way out."News agency Fides said Martinelli had been in Libya for 10 years and was well connected in the country.There are about 100,000 Catholics living in Libya, mostly foreigners employed as nurses.