The secretary of Ukraine's national security council says seven possible causes are under investigation in the plane crash in Iran earlier this week.

Oleksiy Danilov didn't list the possible causes during his remarks to Swedish broadcaster TV4. He said Ukrainian experts are working at the site near Tehran where the Ukrainian jetliner with 176 people on board crashed.

Danilov said that "so far we are happy with our cooperation with the Iranian side and we hope that it continues. It is now a difficult territory, militarized, like the whole world currently, but we hope for our cooperation to continue calmly like this.”

He said Ukraine has 45 investigators in Iran and the experts have access to the plane's debris.

He said he doesn't know if the flight data recorders will be handed over to Boeing, the maker of the plane. "I think rather not than yes, but I think all the countries interested in this situation, all those who suffered, will have access to the black boxes," he said.

Danilov also said the collection of DNA samples is almost complete, after which countries can repatriate victims' remains.