MOSCOW — Further aggravating already tense relations between Russia and Ukraine, the Russian energy giant Gazprom cut off natural gas supplies to its neighbor on Monday, warning that the reduction could diminish the amount of gas flowing to Europe.

The cutoff came after Ukraine missed a Russian-imposed deadline Monday to pay a nearly $2 billion installment for past gas deliveries, with senior officials on both sides exchanging heated remarks blaming the other.

It also coincided with a statement by the Ukrainian president, Petro O. Poroshenko, saying he would present a detailed cease-fire plan this week to end the fighting in eastern Ukraine. But it remained unclear how such a pact would be negotiated, in that Kiev has repeatedly said it would not talk with any separatists who had taken part in the uprising against the central government. For their part, the separatist leaders say Kiev has long ignored their concerns.