(Updates number on board, adds details)

By Patryk Wasilewski

WARSAW, Jan 23 (Reuters) - Several Polish air force commanders were feared killed on Wednesday when their military plane with 19 people aboard crashed on the way back from a conference on aviation safety.

Officials said the Spanish-made CASA 295m transport plane carrying 15 passengers and four crew was approaching a military airport in northern Poland during the evening when the accident occurred.

Colonel Cezary Siemion, a senior defence ministry spokesman, said all aboard the plane were assumed to have been killed. Officials said 19 people were aboard, having earlier said there were 18.

A health official said the wrecked plane had burst into flames.

"Ambulances (at the scene) have turned round, everything went up in flames," Janusz Napiorkowski, head of a nearby military hospital, told dziennik.pl news Web site.

Another military spokesman said the plane could have hit trees on the approach to the airport.

The aircraft was carrying officials taking part in an aviation safety conference and Siemion said senior air force commanders were among the casualties.

"Air force personnel were on board, mostly high-ranking officers," Siemion said.

Defence Minister Bogdan Klich said the incident would be thoroughly investigated. Prime Minister Donald Tusk was due to visit the site of the crash, his office said.

"This is a major disaster," Klich told TVN24. "I'd like to give my sympathies to the families. We share your pain."

Officials said the crash was the worst disaster for the Polish air force since 1973 when 18 people, including two government ministers, were killed in a similar accident. (Reporting by Patryk Wasilewski and Chris Borowski, writing by Adam Jasser; Editing by Charles Dick)

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