* Explosion on pipeline does not disrupt flow to EU
* Possible theories include "act of terrorism" (Updates to add death of secound Russian journalist)
By Pavel Polityuk
KIEV, June 17 (Reuters) - Ukraine said on Tuesday it was treating an explosion on a pipeline carrying Russian gas to the rest of Europe as a possible "act of terrorism", intended to discredit Ukraine as a reliable supplier.
The blast, after Russia cut supplies to Kiev in a price row but continued supplying the European Union, caused no casualties and did not interrupt gas flows but has increased tension as Kiev tries to end an uprising by pro-Russian separatists.
The Interior Ministry issued a statement which described the blast, which sent a plume of dark smoke high into the sky over central Ukraine, as "the latest attempt by the Russian side to discredit Ukraine as a partner in the gas sector".
"Several theories of what happened are being considered including the key theory - an act of terrorism," Interior Minister Arsen Avakov said in the statement, posted on the ministry's website.
"According to local residents, they heard two big bangs just before the explosion which could indicate they were deliberate explosions," he said of the incident in the Ukraine's Poltava region.
The Energy Ministry also suggested there may have been foul play, saying it was "not the first attempted terrorist attack on the Ukrainian gas transportation system."
There was no immediate comment from Moscow or the rebels who rose up in eastern Ukraine, many of them hoping Russia would absorb the region following its annexation of Crimea in March.
Earlier on Tuesday, Prime Minister Arseny Yatseniuk told parliament in Kiev that Moscow had blocked attempts to reach a deal in the long-running dispute over the price Ukraine should pay for Russian natural gas and Kiev's unpaid bills.
"It is part of a plan that envisages a whole series of measures aimed at destroying Ukrainian independence and statehood," he said, listing the annexation of Crimea, "destabilising" of eastern Ukraine and backing of the rebels.
"They still cannot understand that Ukraine is an independent state, and it is no matter of Russia to define where we should go. And we are going in the direction of the European Union," he said.
Moscow has blamed Kiev for the failure to reach agreement, with big differences remaining over the price.
Ukraine has said it will try to restore control of the border with Russia to prevent further violence.
But Tuesday's explosion was far from the violence in east Ukraine, where border guards said 30 servicemen had been wounded in an overnight mortar attack near the border.
The Urengoy-Pomary-Uzhgorod pipeline, which was hit by the blast, is the main transit route for Russian gas to the EU via Ukraine. Police said the blast on the pipeline happened about two metres (six feet) below ground.
Ukrainian state-run gas transport monopoly Ukrtransgaz said there was no disruption to the gas flow. Emergency services said the blast was caused by the pipeline becoming depressurised, though it did not say what had caused that to happen.
In Moscow, the Rossiya-24 channel said its Russian reporter Igor Konelyuk and sound engineer Anton Voloshin were killed when their position was shelled in clashes near the eastern city of Luhansk.
The Russian Foreign Ministry said the deaths showed the "criminal nature" of a military operation launched against the rebels by the Ukrainian government and urged the authorities in Kiev to investigate. (Additional reporting by Denis Pinchuk and Alissa de Carbonnel in Moscow; Writing by Timothy Heritage; Editing by Robin Pomeroy and Sonya Hepinstall)