MOSCOW — The Ukrainian government declared a moratorium Friday on repaying about $3.5 billion in debt to Russia , introducing new strains to an already poisonous relationship. Ukraine ’s cabinet prohibited further payments to Russia unless Moscow agrees to join a restructuring program that has been accepted by commercial creditors — a step President Vladimir V. Putin of Russia has rejected — or a court orders Ukraine to pay up. Ukrainian officials said that paying the Russians in full now, after commercial creditors agreed last summer to a 20 percent write-down and long delays in repayments, would be illegal and run counter to Ukraine’s $17.5 billion International Monetary Fund bailout. “If we were to repay this bond in full, it would mean we failed to meet the terms of the I.M.F. and the obligations we made under our restructuring,” Natalie A. Jaresko, Ukraine’s finance minister, said in a telephone […]