Iran's government has denied a "cover-up" after it took days for the armed forces to admit a Ukrainian airliner was shot down by mistake last week.The comments came after a second night of demonstrations in Tehran against the authorities over the air disaster, according to videos shared on social media.The Ukraine International Airlines Boeing 737 was shot down shortly after it took off from Tehran before dawn on Wednesday, killing all 176 passengers and crew on board.The Kiev-bound airliner was brought down hours after Iran had launched a wave of missiles at US troops stationed at Iraqi bases in retaliation for the killing of a top Iranian general.The Islamic republic initially denied Western claims based on US intelligence that the passenger plane had been struck by a missile before admitting it on Saturday.MUST READ | Iran thought the Ukrainian passenger plane it shot down was a US military plane"In these sorrowful days, many criticisms were directed at relevant officials and authorities," said government spokesman Ali Rabiei."Some officials were even accused of lying and a cover-up but, in all honesty, that was not the case," he said in remarks aired on state television."Lying is intentionally and knowingly faking the truth. Lying is covering up. Lying is knowing a fact and not expressing it or twisting the truth."Rabiei said all details provided by officials prior to Saturday's admission had been based on the information available to them at the time.READ | 'Commander-in-chief, resign, resign' - Iranians demand Khamenei to step down over plane shooting"All of those who expressed opinions on those days, at the peak of America's psychological war against the Iranian nation... did so based on existing information at the time."'Thorough investigation'President Hassan Rouhani promised a "thorough investigation" into the air disaster in a telephone call with Sweden's Prime Minister Stefan Lofven on Sunday night, his office said.The majority of the people on Flight PS752 were Iranians and Canadians, including dual nationals, and other victims were Ukrainians, Afghans and Britons.There were seven Swedish citizens as well as 10 people who reside in the EU member country."We must strive to ensure that such a shocking incident is not repeated anywhere in the world," Rouhani said, quoted in a statement published on his government's website.Rouhani noted that the disaster occurred at a time of heightened tensions in the region after a US drone strike killed Iranian general Qasem Soleimani in Baghdad on January 3.ALSO READ | Downing of Ukraine plane: The world demands full accountability from Iran"We must all join hands to bring security back to the region and allow peace to prevail," the Iranian president said.Iran has invited experts from Canada, France, Ukraine and the United States to take part in the investigation into the air disaster.Despite footage from the site of disaster appearing to show bulldozers clearing the area, the Revolutionary Guards' top commander denied any evidence had been tampered with."We didn't touch anything," Major General Hossein Salami told parliament on Sunday."We didn't move the wreckage of the aircraft, we didn't change the scene, we didn't move the air defence system, and we didn't (alter) the radar readings," he said.