KYIV, Oct 5 (Reuters) - Ukraine will increase its steel output to 6.5 million metric tons in 2023, but could double that if its sea ports were unblocked, the head of the Ukrmetalurgprom steel producers union said on Thursday.

Ukraine was ranked 25th among steel producing countries in 2022, when Russia's invasion dragged its output down by 71% to 6.3 million tons from 21.4 million tons, according to the World Steel Association.

Two huge steel mills were largely destroyed during a months-long Russian siege of Mariupol, a city and port on the Azov Sea coast.

In the Black Sea, the movement from Ukraine's ports is limited to a temporary humanitarian corridor after Russia blocked the U.N.-backed grain deal that had been in place for a year.

"The Ukrainian steel industry has been hit probably the hardest (by the invasion). In 2022 we lost a number of plants and first of all these are the Mariupol plants," Ukrmetalurgprom head Oleksandr Kalenkov told Reuters in an interview.

Before the Russian invasion in February 2022, Ukraine operated 10 steel mills with a combined annual capacity of 25.3 million tons, and its six surviving mills can produce about 17 million tons, he said.

Kalenkov said that in the first eight months of 2023, production rose by 5% thanks to reconstruction programmes and large orders from Ukraine's armed forces, but opening the Black Sea ports for exports was vital.

Railway tariffs have doubled over the year, he said, and the blocking of seaports has deprived the industry of the cheapest way to export steel.

"If we don't have the seaports open, our industry will not survive and everyone else will have problems after us. If there is shipping, we will be able to double production," Kalenkov said.

Ukraine's steel sector has traditionally exported about 80% of the steel it makes, Kalenkov said.

"We have always been very dependent on exports - for the last 25-30 years exports of steel products have been about 80% and there is no other country in the world that is so dependent on exports," he said.

Ukraine's domestic market before the war was about 5 million tons annually, and in the first year of the war it shrank to 2 million tons. But reconstruction programmes and a surge in demand from the military caused domestic demand to jump in 2023.

"We have an export share of 53% (in 2023). This year, the domestic market has lent a shoulder and due to this our companies are increasing production," Kalenkov said.

In 2014, when Russia annexed Crimea, Ukraine lost control of some territories in its industrial east, where about 30% of its steelmaking capacity was located.