Bulgaria's capital Sofia will launch a new 6.5-kilometre section of its subway network by the end of August, the transport ministry said.

Eighteen new trains have been ordered to service the new section, the ministry said in a statement on its website.

"The first 12 trains will be put into operation by the end of August, and the remaining six by the end of November," transport minister Ivailo Moskovski said in the statement.

The new trains cost 153 million levs ($98.4 million/78.2 million euro), as the money has been provided under the EU-funded operational programme Transport.

The trains are manufactured by a tie-up between Russian factory Metrovagonmash and Japan's Hitachi. They are 25% more energy efficient than the ones that service the subway network at present.