The headquarters of Ukraine’s national energy company Naftogaz have been raided by security forces carrying out a criminal investigation.Witnesses spoke of seeing armed and masked men entering the building earlier this morning. The reason for and circumstances of the operation remain unclear. A Naftogas official said the officers were seeking original copies of a supply contract signed with Russia’s energy firm Gazprom. He was quoted as saying that without these documents Naftogaz could not bring gas through customs and could not fulfill its contractual obligations. Russia and Ukraine have previously struggled to resolve rows over gas supply and terms of new contracts, though it is not clear if this morning’s raid is in any way connected to these disputes.