Alliance members Britain and Canada -- which lost 63 of its citizens in the disaster -- said there was intelligence to suggest that Iran shot down the Ukraine International Airlines Boeing 737 with a missile, possibly by mistake. Tehran has denied the claim.

"I will not go into details about our intelligence but what I can say is we have no reason to not believe the reports we have seen from different NATO allied capitals," Stoltenberg said.

"They have expressed concern about information indicating that the plane may have been downed by Iranian air defence systems."

Stoltenberg added his voice to growing international calls for a proper investigation of Wednesday's disaster, which came just hours after Tehran hit bases in Iraq housing US troops with missiles in retaliation for an American drone strike that killed a top Iranian general.

He said the missile concerns were "exactly why we need thorough investigation, that's exactly why we need to establish all the facts and that's exactly why it's so important we have full cooperation from the Iranian side in such an investigation".

Stoltenberg was speaking as he joined EU foreign ministers for emergency talks called in response to the Iran crisis, though fears of all-out war between the US and Iran have subsided since the start of the week.

pdw/dc/klm

BOEING