United States President Donald Trump implied that the Ukrainian airliner that recently crashed in Tehran may have been shot down by "mistake' during a press briefing in Washington DC on Thursday, at an unveiling event for a proposal for major changes to environmental law.

Trump said that he had his "suspicions" about what caused the Ukrainian International Airlines' Boeing 737-800 to crash in Tehran.

"I have my suspicions," stated Trump, adding "it's a tragic thing, but somebody could have made a mistake."

"It was flying in a pretty rough neighbourhood and somebody could have made a mistake. Some people say it was mechanical, I personally don't think that's even a question, personally," he added.

The Ukrainian International Airlines' Boeing 737-800 crashed shortly after take-off near Tehran's Imam Khomeini International Airport on Wednesday morning, killing all 176 people on board.

The Ukrainian International Airlines Boeing 737-800 heading to Kiev crashed minutes after take-off near Tehran's Imam Khomeini International Airport on Wednesday morning, killing all 176 people on board.

Iran denies the accusations, with Head of Iran's Civil Aviation Organisation Ali Abedzadeh slamming the "illogical rumors" that Tehran missile had struck the Ukrainian Boeing 737, adding that it was "scientifically" impossible.