"To commemorate the victims, the President orders that the state flags of Ukraine be flown at half-mast on the headquarters and offices of state authorities, local governments, state-owned enterprises, institutions and organisations," President Vladimir Zelenski said in a statement, Efe news reported.
On Wednesday, a Kiev-bound Boeing 737 of Ukraine International Airlines carrying 179 people, mainly Ukrainian and Canadian passengers, crashed near the Iranian capital after take-off, killing all on-board.
"Of course, the priority for Ukraine is to establish the causes of the crash," the President said, adding: "We will certainly find out the truth."
"To this end, a detailed and independent investigation will be carried out in accordance with the international law," he added
The President hoped that the Ukrainian specialists who arrived in Iran on Thursday will join the team looking into the accident by analysing the black boxes of the plane.
Experts will also carry out the work of identification and repatriation of the mortal remains of Ukrainian passengers.
The President called on Ukrainian citizens to avoid trips to the region and not to speculate on the reasons for the accident until the official results of the investigation were announced.
"This is not a topic for hype, likes in social networks, sensationalism and conspiracy theories. We need patience, endurance, and wisdom.
"And also this terrible story should teach us all, every citizen of Ukraine, and every world leader, to value human lives. It should be done before they are taken so quickly and painfully," Zelensky said.
Minutes after take-off from Tehran's Imam Khomeini airport, it crashed while attempting to return to the airport, according to investigators.
The reasons for the sudden crash remain unknown.
The plane was manufactured in 2016 and passed its last technical review two days before the fateful flight.
( With inputs from IANS )