The crash of a Ukrainian International Airlines jet shortly after takeoff from Iran's capital early Wednesday that killed all 176 passengers and crew raises questions and could add to problems for Boeing, the plane's builder.

The Boeing 737 NG, a single-aisle plane designed for short- to medium-length flights, left Teheran after 6 am, bound for Kiev.

The flight crew didn't make a "mayday" call from the cockpit to alert the airport that the plane had mechanical difficulties. One aviation expert said it appeared the jet was "not intact" before the crash, The Washington Post reported.

"The wreckage pattern was very consistent with a plane that was not intact when it hit the ground," Todd Curtis, a safety analyst for AirSafe.com and a former Boeing safety engineer, told the newspaper.

The crash could have been the result of an in-flight breakup of the plane, an in-flight explosion, midair collision, structural failure, an external strike or a major on-board equipment malfunction, he said.

A video posted on social media appeared to show "a large amount of fire" that could have been caused by an "uncontrolled engine failure" prior to the crash, John Cox, a former pilot and airline safety consultant, told the newspaper.

International rules say the US should participate in the crash investigation because Boeing is an American company, but Iranian officials said they won't cooperate with US investigators or the jet's builder, according to media reports.

The plane's "black box" — the cockpit voice and flight data recorder crucial to determining the cause of a crash — has been recovered, Iranian state broadcaster IRIB reported.

"We will not give the black box to the manufacturer (Boeing) or America," Ali Abedzadeh, director of Iran's Civil Aviation Authority, told that nation's news agency.

The Ukrainian embassy in Teheran withdrew its previous statement ruling out terrorism or a rocket attack as possible causes for the crash.

An early report by Iranian media blamed engine failure. Rescue crews rushed to the scene, but the site was ablaze and they couldn't assist, the head of Iran's emergency medical services told state media.

The Ukranian airline said the plane had been in service for about 3.5 years and was last serviced Jan 6.

At least 16 of those killed were children under age 10. Most of the passengers were Iranian, but there were also Canadians and Ukrainians on the flight as well as a few citizens of Britain, Germany and Sweden.

"This is a tragic event and our heartfelt thoughts are with the crew, passengers and their families," Boeing said in a statement.

The Ukrainian jet crash came after Iran's Islamic Revolutionary Guard fired rockets at two bases with US troops in Iraq. Iran's government said the action was retaliation for the US killing of General Qassem Soleimani in a drone attack. He commanded Iran's Quds Force.

The Boeing 737 NG involved in the crash is the predecessor to the 737 MAX and isn't equipped with the automated anti-stall system implicated in the Indonesian and Ethiopian crashes that led to the MAX's worldwide grounding in March 2019.

In a reversal from previous statements, Boeing said Tuesday that it now recommends flight simulator training for 737 MAX pilots, as the aircraft builder struggles to get the plane back in the air following two fatal crashes. Previously, Boeing said a laptop computer course would prepare pilots for the updated flight system.

"It's not clear that the Federal Aviation Administration agrees with Boeing's recommendation," Robert Mann, president of R.W. Mann &Co, an aviation consulting firm, told China Daily. "It's not clear that a curriculum can be developed quickly enough to allow the limited number of MAX simulators to handle the required number of pilots."