KYIV, July 9 - Naftogaz Ukrayiny, the national energy company, sued Gazprom to force the Russian company pay higher tariffs for gas shipments via its gas pipeline system in 2018-2019, a senior Naftogaz official said Saturday.

The lawsuit was filed with the Stockholm court of arbitration and is estimated by Naftogaz at $11.6 billion without adding interest rate charges.

Naftogaz asked Gazprom in March to review the gas transit tariffs in line with European practices, but the Russian company had flatly refused, triggering the latest lawsuit.