Ukrainian officials say a blast hit a pipeline used to transport Russian gas to Europe, a day after Moscow cut supplies to Ukraine. Elsewhere, a Russian TV reporter was killed in fighting in the country's east.
Ukrainian police officials said the Urengoi-Pomary-Uzhgorod Pipeline, also known as the Trans-Siberian Pipeline, was damaged in the northeastern Lokhvytsia region.
Nobody appeared to be injured and Ukraine's state gas company, Ukrtransgaz said the explosions did not appear to affect the westward flow of Russian gas.
The 4500-kilometer (2800-mile) link connects gas fields in Russia to western Ukraine, and from there, to Russia's western and central European clients.
Ukraine state emergencies service said a puncture or a loss of pressure in a seal appeared to have caused the blast.
The government in Kyiv, which faces a rebellion by pro-Russian separatists in the east, says it could have been a terrorist attack.
"It is not the first attempted terrorist attack on the Ukrainian gas transport system," the Energy Ministry said in a statement. There were two explosions on transit pipelines in Ukraine's west in May.
Tuesday's explosion comes a day after Russia cut off natural gas supplies to Ukraine, after it missed a Moscow-set deadline to pay part of its outstanding bill.
Russia said on Monday that Ukraine must now pre-pay for gas.
"Today (Monday) at 10 a.m. Moscow time (0600 UTC), Gazprom in full accordance with contractual obligations, switched to pre-payment gas deliveries for Naftogaz Ukraine," the Russian gas giant said in a statement.
Negotiators from both sides had met on Sunday night, but were unable to resolve a dispute over unpaid debts and the price Ukraine pays for Russian gas.
Russian journalist killed
A Russian reporter was killed in eastern Ukraine on Tuesday, after his position was shelled during clashes between Ukrainian forces and pro-Russian separatists, according to Moscow state television.
Rossiya-24 television said 37-year-old Igor Kornelyuk died in hospital after coming under fire while reporting on heavy fighting in Ukraine's easternmost province.
The fate of Kornelyuk's colleague, Anton Voloshin, was "unknown," Rossiya-24 said.
Kornelyuk received serious stomach wounds after being hit by shrapnel from what appeared to be a mine or a grenade, said the chief doctor at the Luhansk region's Clinical Hospital.
"He was unconscious when he arrived and died on his way to the operating room," said the doctor, Fedir Solyanyk.
A separatist spokesman told the AFP news agency the whereabouts of Voloshin and that of 15 rebel fighters who were with the Russian TV crew were unknown.
Korneyluk's death is the second confirmed fatality of a reporter in eastern Ukraine since fighting broke out there in mid-April, after Italian photographer Andrea Rocchelli was killed in Slovyansk in late May. Rocchelli's Russian translator, Andrei Mironov, also died.
The crisis in Ukraine began last year amid mass protests against former president Viktor Yanokovych, who abandoned a key EU agreement in favor with closer ties with Moscow. After his overthrow in February, Russia annexed the Crimea region in March, and the uprising in Ukraine's east began in April.
jr/jlw (Reuters, AFP)