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.