With monsoon rains causing thick mud and slippery hillsides, recovery efforts at the site of Pakistan's worst-ever plane crash have been mostly halted Thursday.

Helicopters too have not been able to fly in the heavy rain and low clouds, said a spokesman for the Capital Development Authority, which helps deal with emergencies.

Before work was halted by the rain, army troops and rescue workers picked through the remains and were joined by relatives of those killed, who are desperate to find evidence of their loved ones.

An Associated Press Television News cameraman in the hills saw relatives of passengers working with soldiers at one crash site, where the undercarriage of the jet had come to rest. They had collected several body parts in small bags.

The Pakistani government declared Thursday a day of mourning for the 152 people on board the Airbus 321 who were killed in Wednesday morning's crash.

The flight, on its way from the southern port city of Karachi, crashed during stormy weather into a steep and heavily-wooded hillside outside Islamabad shortly before it was due to land.

The civil aviation authority said the plane had been ordered to take an alternate approach to the runway, but had veered off course.

Finding out why will be a key task of the investigation team, said Riazul Haq, director general of the Civil Aviation Authority.

"The fact remains it flew where it should not have done," he said.

Terrorism has been all but ruled out, with the rains and fog being the most likely cause of the crash.

Raheel Ahmed, a spokesman for the Airblue airline that operated the jet, said the plane had no known technical issues. He also noted the pilots did not send any emergency signals. Airbus said it would provide technical assistance to the crash investigators.

With recovery work mostly halted, there are worries the monsoon rains could damage or wash away any evidence of what caused the crash, the investigation team's head, Khawaja Abdul Majeed, told Dawn News television Thursday.

"Time is very precious," he said. "We have to collect evidence as soon as possible, so we don't have much time."

The plane's "black box" flight data recorders, which could hold precious information about what went wrong, have yet to be recovered.

The federal information minister said late on Wednesday rescue workers had been able to recover 115 bodies. A worker outside Islamabad's largest hospital told AP that 59 bodies had been identified and taken away by their relatives.

But identifying the rest of the bodies could take days of DNA testing, since most of the bodies were torn apart and burned in the crash.

Relatives and friends of those killed slept outside the hospital overnight, hoping to receive bodies. Most were still there Thursday morning, though few corpses were released.

With reports from the Associated Press