By Andrew Osborn and Pavel Polityuk
MOSCOW/KIEV (Reuters) - Russia said on Tuesday it would cut off gas supplies to Ukraine and threatened to halt coal deliveries, ratcheting up a dispute over a power blackout in Crimea at a time when a ceasefire between Kiev and separatist rebels is fraying.
Russian Energy Minister Alexander Novak accused the Ukrainian authorities of deliberately refusing to help rebuild power lines to Russia-annexed Crimea, which were blown up by unknown saboteurs over the weekend.
The peninsula of 2 million people is relying on emergency generators to meet its basic power needs, with pro-Ukrainian activists, including nationalist battalions and ethnic Tatars, preventing repairs.
The Ukrainian government dismissed as "absolutely groundless" suggestions, made by two Ukrainian lawmakers to Reuters, that Kiev might be tacitly backing the activists.
The row could fan tensions between Ukraine and pro-Russian fighters in the east, where a recent spike in ceasefire violations is threatening to shatter a truce after two months of relative calm.
Faced with a ban on imports of its food by Moscow for supporting sanctions against Russia, Ukraine said it would hit back with reciprocal trade bans and may also block goods shipments to Crimea. It has urged Western countries not to ease pressure on the Kremlin, fearing that cooperation over fighting Islamist militants in Syria could lead to a thaw in ties.
"Today or tomorrow gas deliveries will be stopped because of lack of advance payment," Novak told Vesti FM radio station, referring to a dispute that pre-dates the blackout row, but which has come to a head at the same time.
Novak said Ukrainian authorities were not doing what was needed to allow repair crews to restore power to Crimea "out of some kind of political motivation", calling their inaction a crime.
"There are different options, political ones, economic ones," said Novak, when asked how Russia could retaliate. "Russia delivers coal to the Ukrainian energy sector. We could, and maybe in this situation we need to, take a decision about halting deliveries."
A Ukrainian lawmaker close to the circle of Prime Minister Arseny Yatseniuk said of the Crimea blackout: "This was all done with the tacit consent of the country's whole leadership."
"What is happening on the Crimean border is not so much pressure on Russia as a demonstration that we are prepared to respond strongly to events in the east and in Crimea," the lawmaker added. "It's a demonstration that Russian pressure on Ukraine could swing back on Russia like a boomerang. It's a demonstration that we also have trump cards."
But the Ukrainian government rejected suggestions of inaction. "This is an absolutely groundless assertion and the quick reaction of the government regarding repairs on the power lines and its guaranteeing of security for people in the emergency zone is proof of this," it said in emailed comments.
In a further sign of rising tensions, Kiev also accused Russia of an aggressive troop build-up in Crimea by sending two more highly-trained battalions to the area on Tuesday.
Last week the Organization for Security and Co-operation in Europe warned the ceasefire in the east was "very shaky", with both sides regularly using artillery despite promises to withdraw heavy weapons from the frontline.
NOT FREEZING IN WIND
Four electricity pylons were blown up over the weekend and Tatar activists say they will only allow Ukrainian engineers to repair one of them, which supplies power to a southern pocket of the country outside Crimea that also lost power.
Tatars, a Muslim people with a long history in Crimea, who opposed Russia's annexation in March 2014, had already imposed an informal economic blockade on the supply of goods to Crimea by setting up road blocks to the area in September.
"Ukrenergo are allowed access to the one pylon, they have started their work," Lenur Islamov, the leader of the Tatar blockade, told TV channel 112 on Tuesday, referring to Ukraine's state energy supplier. "The other pylons are under our control and access is closed."
He said the Tatars welcomed the decision by the Ukrainian government to restrict the flow of goods to Crimea, meaning the activists would not have to man road blocks in icy weather.
"We're tired of freezing in the wind," he said. "Every driver stops and gets angry with us, saying they're right by law."
Russia is laying cables under the Black Sea to supply Crimea with electricity, cutting its reliance on Ukraine, and on Tuesday announced it had brought forward the deadline for the first phase of the project by two days to Dec. 20.
As a stopgap measure, Sergei Aksyonov, the Kremlin-backed head of Crimea, said that Russia was sending emergency diesel generators to the area, including 300 via a federal rescue agency.
"The situation in Ukraine is becoming political in nature. Certain figures, former Crimeans, are trying to push some political demands, making trade-offs," he wrote on Facebook. "Anyone who harms our citizens will be punished. I make it clear to everyone: Crimeans will not sell their homeland."
Aksyonov sacked Crimea's energy minister on Tuesday, accusing him of mishandling the situation, following media reports that doctors at some hospitals were having to carry jerry cans full of diesel to power generators.
(Additional reporting by Alessandra Prentice and Natalia Zinets in KIEV and Jack Stubbs in MOSCOW; Writing by Matthias Williams; Editing by Philippa Fletcher and Pravin Char)