Russia will completely or partially cease gas deliveries to Ukraine if its debts continue unpaid, Russian President Vladimir Putin wrote in a letter to European leaders Thursday. A shutoff would devastate Ukraine’s already weak economy and could reduce flows of the important heating and electricity fuel to the rest of Europe.

It has long been assumed Russia might use a supply cutoff as a tool in its negotiations with the West over Ukraine – it has twice shut off the gas in winters past after Ukraine amassed large debts to its primary supplier. But Thursday’s warning is by far the Kremlin’s most direct and explicit in the latest round of tensions over Ukraine’s future. It promises to add a new level of urgency to Ukraine and broader Europe’s efforts to diversify its energy mix.

“Undoubtedly, this is an extreme measure,” Mr. Putin wrote in Thursday’s letter, acknowledging that gas piped through Ukraine intended for Europe might also be interrupted. A sudden drawdown of Russian supply would also make the fall and winter heating season difficult for Ukraine, Putin wrote. “However, the fact that our European partners have unilaterally withdrawn from the concerted efforts to resolve the Ukrainian crisis, and even from holding consultations with the Russian side, leaves Russia no alternative.”