* Number of casualties expected to rise
* Plane crashes just before landing in bad weather
* Relatives gather at Tehran's Mehrabad airport
(Adds details, quotes)
By Hossein Jaseb and Ramin Mostafavi
TEHRAN, Jan 9 (Reuters) - At least 70 people were killed and 35 injured when an Iranian Boeing 727 <BA.N> passenger plane crashed in bad weather in northwestern Iran on Sunday, the head of the Iranian Red Crescent said.
Mahmoud Mozafar told Reuters that many of the injured were in a bad condition. He said 106 people, 12 crew members and 94 passengers, were on board the Iran Air plane when it crashed.
Another Red Crescent official, Heidar Heidari, told the state news agency IRNA, that the "death toll is expected to increase".
Relatives of people aboard gathered at Tehran's Mehrabad airport to get information from authorities, a Reuters cameraman said.
Earlier, Iranian media gave different accounts of the number of people on board the plane, which some officials said crashed just before landing at the airport in the city of Urumiyeh, located about 1,000 km (625 miles) northwest of the capital in a mountainous area near the border with Turkey.
One official said 50 people were rescued and that rescue operations were being hampered by snow and fog in the area.
Mozafar told state television "the plane was smashed into pieces but did not explode".
State television showed footage of the wreckage of the plane, which appeared to have broken into several sections on impact. Huge gaps were rent in the fuselage, parts of which were open to the night air as snow fell.
Shahrokh Nioushabadi, a spokesman for Iran's national airline, Iran Air, told the semi-official Mehr news agency that two children were among the passengers.
State television said the Iran Air plane was en route from Tehran to Urumiyeh.
Iran has suffered a string of crashes in the past few decades. U.S. sanctions against Iran have prevented it from buying new aircraft or spare parts from the West.
The last major air crash in Iran was in July 2009 when a Caspian Airlines Tupolev aircraft bound for Armenia caught fire in mid-air and crashed into farmland near the city of Qazvin, killing all 168 people on board.
One of the country's worst air accidents happened in February 2003 when an Iranian Ilyushin-76 troop carrier crashed in southeast Iran, killing all 276 Revolutionary Guard soldiers and crew aboard.
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