Ukraine said on Tuesday (local time) it was treating an explosion on a pipeline carrying Russian natural gas to the rest of Europe as a possible "act of terrorism".
The government, which is facing a rebellion by pro-Russian separatists in east Ukraine, said the blast in central Ukraine - one day after Russia cut gas supplies to Kiev in a pricing dispute - did not disrupt gas flows to the European Union.
But Interior Ministry Arsen Avakov said: "Several theories of what happened are being considered including the key theory - an act of terrorism."
"According to local residents, they heard two big bangs just before the explosion which could indicate they were deliberate explosions."
The Energy Ministry also suggested there may have been foul play, without speculating who might have been behind the blast.
"It is not the first attempted terrorist attack on the Ukrainian gas transportation system," the ministry said in a statement, referring to two explosions on transit pipelines in the western part of the country in May.
Tuesday's explosion was in the Poltava region, which is far from the violence that has rocked east Ukraine, where the pro-Russian separatists have risen up against central rule.
The Urengoy-Pomary-Uzhgorod which was hit on Tuesday is the main transit pipeline carrying Russian gas to the European Union via Ukraine.
Kiev's relations with Moscow are in crisis following Russia's annexation of the Crimea region. Russia stopped supplying Ukraine with gas on Monday but Kiev said it was guaranteeing the gas flow to the European Union.
Police in Poltava region said the blast was at 2.20pm local time (11.20pm NZ time) and hit the pipeline about two metres below a field. There were no casualties.
Video footage showed smoke pouring high into the sky.
Ukrainian state-run gas transport monopoly Ukrtransgaz said there was no disruption the gas flow and a source at Russian gas producer Gazprom said: "Exports have not been cut. There is a parallel pipeline."
The emergency services said the blast was caused by the pipeline becoming depressurised, though it did not say what caused it to become depressurised.