Iran is facing mounting pressure to explain the destruction of a civilian airliner near Tehran hours after Iranian forces launched missile strikes against US forces.

Three Britons were among 176 people killed when an Ukrainian International Airlines Boeing 737 plunged into a nosedive and exploded shortly after take-off from Tehran’s Imam Ayatollah Khomeini International Airport on Wednesday morning.

Iran dismissed speculation that the aircraft had been downed by a missile but said it would not hand over the black boxes from the aircraft to Boeing, in an unusual move likely prompted by high tensions with the United States.

Earlier Ukraine withdrew an initial statement attributing the crash to engine failure and ruling out a terror attack, sparking an international scramble to investigate the crash.

The crash came three and a half hours after Iran fired a barrage of ballistic missiles at US bases in Iraq in what it said was revenge for the assassination of general Qassim Soleimani.

Iran’s military said it had fired 22 missiles at Iraqi bases housing US troops in Erbil and Ain al-Asad. No US or Iraqi troops were killed in the half-hour bombardment.

Leaders in both Tehran and Washington played down the prospect of further armed conflict between the adversaries. In a televised address Donald Trump claimed that Iran "appeared to be standing down" and said he would impose further economic sanctions, but did not mention further military action.

Although Iran’s Supreme Leader, Ayatollah Ali Khamenei, termed the strikes a “slap in the face” for America and vowed to continue a campaign to oust US forces from the region, the Iranian foreign minister, said the strikes "concluded" Iran's military response to the assassination of Soleimani.

The Ukrainian International Airlines Boeing 737-800 bound for Kyiv took off shortly after 6 AM Tehran’s Imam Khomeini International Airport early on Wednesday morning.