Officials believe a passenger jet may have swerved off course in heavy rain and cloudy conditions when it crashed into the hills near Islamabad, Pakistan, killing all 152 people on board Wednesday morning.

The doomed Airblue Airbus 321 took off from an airport in Karachi, more than 1,500 kilometres south of the Pakistani capital.

Authorities were searching the wreckage for the black box flight data recorder and have launched an investigation into the crash, which occurred in the Margalla hills that surround Islamabad, about 15 kilometres from the airport.

"The situation at the site of the crash is heartbreaking," said Imtiaz Elahi, chairman of Pakistan's Capital Development Authority, which handles emergencies. "It is a great tragedy, and I confirm it with pain that there are no survivors."

It's not clear what caused the tragedy, but Defense Minister Chaudhry Ahmed Mukhtar said terrorism is not suspected. According to the Pakistan Airline Pilot Association, the plane may have drifted off course due to poor weather conditions.

The death toll is the largest of any plane crash in Pakistani history, prompting Pakistan's government to declare Thursday a national day of mourning.

"There is nothing left, just piles and bundles of flesh," said rescue worker Murtaza Khan. "There are just some belongings, like two or three travelling bags, some checkbooks, and I saw a picture of a young boy. Otherwise everything is burned."

The majority of the victims were believed to be Pakistani citizens, though the U.S. Embassy in Islamabad confirmed that two Americans were among the dead.

In Ottawa, a Foreign Affairs spokesperson said the department has no indication that any Canadians were involved in the crash. But any Canadians who believe that their friends or family members were on board the plane are asked to contact the DFAIT Emergency Operations Centre at 1-800-267-6788.

Five Pakistani members of a Markham, Ont., family were killed in the crash.

"That moment, I'm trying to make myself understand that," Furquan Khan told CTV News. "No, this couldn't be my brother and my sister, and my family."

Khan's older brother was on board the plane along with his wife and their three children, after they had booked a last-minute vacation.

"It's really hard for me to believe that my family is gone," Khan said.

At the crash site, authorities described a scene of utter devastation, with plane wreckage and human remains scattered throughout.

"I'm seeing only body parts," Pakistan Red Crescent rescue worker Dawar Adnan told The Associated Press in a telephone interview from the crash site. "This is a very horrible scene. We have scanned almost all the area, but there is no chance of any survivors."

Adnan said rescue efforts were being hampered by persistent fires that had emerged as a result of the crash.

By the time rescue workers left the site Wednesday night, authorities reported that 115 bodies had been recovered.

No known cause of crash

Flight ED202 took off from the Karachi airport at 7:45 a.m. and was supposed to land in Islamabad about two hours later, civil aviation official Pervez George said.

The plane was preparing to land "when it lost contact with the control tower, and later we learned that the plane had crashed," George said.

It was not immediately clear what caused the crash, he added, though weather reports suggested that the plane was trying to land during a period of difficult, cloudy weather.

Saqlain Atlaf was on a family outing in the hills outside of Islamabad when the passenger jet crashed.

"The plane had lost balance, then we saw it going down," Atlaf told Pakistan's ARY news channel.

Other witness accounts suggested the plane was flying a very low, unsteady course right before it crashed.

Earlier Wednesday, Airblue spokesperson Raheel Ahmed said the Airbus involved in the crash was only eight years old and had no known technical issues.

Airbus said it would assist Pakistani authorities in their investigation.

According to figures from Airbus, the downed plane had 34,000 flight hours and had been used in 13,500 flights.

The Airblue airline is based out of Karachi and operates domestic flights within Pakistan, as well as flights to the United Arab Emirates, Oman and the United Kingdom.

With a report from CTV Toronto's Michelle Dube and files from The Associated Press