President of the United Nations Development Program (UNDP) Gray Lewis called collapse of a 17-story high-rise structure in Tehran a tragedy.
"Our thoughts are with friends and families of the fire fighters who have died following the fire and building collapse in Tehran, Iran,” he twitted.
He also appreciated London Firefighting for expressing sympathy at the tragedy.
The 17-story structure crumbled after the fire engulfed the top floors of the building in downtown Tehran as scores of firefighters battled the blaze.
The Fars news agency and the IRIB news website said at least 30 firefighters were feared to have been killed in the incident but officials did not confirm any deaths.
The head of Tehran's emergency services told IRIB that at least 70 people have been injured and 23 hospitalized.
Tehran Mayor Mohammad Baqer Qalibaf said between 20 to 25 people are believed to have been trapped under the rubble.
Officials have not given the exact number of casualties so far.