While airlines in the former Soviet Union generally have poor safety records, Ukraine International Airlines said on its website that its safety was audited and met F.A.A. standards for code-sharing flights with foreign partners. It had not previously experienced a fatal crash, according to a list of Ukrainian aircraft accidents compiled by the Flight Safety Foundation.

The 737-NG is one of the world’s most widely used airliners; more than 7,000 have been built since 1998, and the plane has logged more than 250 million hours of flight time and fewer than a dozen fatal accidents.

But Boeing’s engineering has come under scrutiny over the past year, after two crashes involving a different model of jet, the 737 Max. Those accidents, which were caused in part by new software on the plane, have plunged Boeing into its biggest-ever crisis. The Max has been grounded worldwide since March, and the company’s chief executive was fired last month.

As the company struggles to get a fix for the Max approved by regulators, new safety risks have recently emerged with the plane. The company may also need to assess those risks in the 737-NG family, which includes the 737-800. Boeing shares were down almost 2 percent on Wednesday.

If the crash in Tehran turns out to be a mechanical failure, raising questions about the 737-NG, the results could be devastating for Boeing. “We are aware of the media reports out of Iran and we are gathering more information,” Boeing said in a statement.

Reporting was contributed by Andrew Kramer, Natalie Kitroeff, James Glanz, Malachy Browne, Nilo Tabrizy, Farnaz Fassihi, Christiaan Triebert and Sarah Kerr.