WARSAW, Poland - A military plane carrying officers home from a flight-safety conference crashed in a forested area in northwestern Poland, killing all 20 people on board, the prime minister said early Thursday.

Prime Minister Donald Tusk said the victims of the crash, which occurred as the plane was about to land Wednesday evening, included a brigadier-general and a colonel and was a "huge loss for the Polish air force.''

"Soldiers, husbands and fathers have died, and that is the most tragic result of this catastrophe,'' said Tusk, who had rushed to the site of the accident.

The plane carrying 16 passengers and four crew members was approaching an air force airstrip at Miroslawiec shortly after 7 p.m. when it crashed.

The aircraft, a Spanish-built CASA C-295M military transport plane, was about 3 kilometres from the airstrip when it clipped trees on its approach, crashed in a wooded area and burst into flames, officials said.

Tusk said rescue workers were still looking for the black box in their effort to clarify what went wrong.

The plane had more people on board when it took off from Warsaw, but had already landed at three other military airports -- in Powidz, Poznan and Krzesiny -- to return soldiers to their home bases. It had been scheduled to make one more stop, in Swidwin, before returning to base in Krakow.

Polish media described the accident as the worst military disaster in more than three decades, and President Lech Kaczynski, on a visit to Croatia, was cutting short his trip to return to Poland on Thursday, a spokesman said.

The Polish parliament postponed its planned daily session Thursday in the wake of the accident. A Mass was planned at Warsaw's military church later in the day.

Among those killed were Brig. Gen. Andrzej Andrzejewski, commander of an air brigade based in Swidwin, as well as the commander of the Miroslawiec air base, Col. Jerzy Pilat, said Defense Minister Bogdan Klich, who rushed to the site of the accident along with the prime minister.

Most of the victims were officers from the Swidwin base, TVN24 television said.

A Polish military expert, Grzegorz Holdanowicz, said it was the first air disaster involving a CASA C-295M, a plane he called one of the safest in the Polish air force. The Polish military also uses the plane type in Iraq and in Afghanistan, where it supports the U.S.-led operations.

The pilots of Wednesday's flight were from a transport squadron based in Balice, near Krakow, who had flown in Iraq and Afghanistan, the squadron spokesman, Capt. Piotr Jaszczuk, said.