KIEV (Reuters) - Ukraine's Acting President and Prime Minister offered some of their strongest pledges yet on Friday to strengthen constitutional rights to use the Russian language in a bid to defuse separatist protests.

In a joint televised address, acting President Oleksander Turchinov and Prime Minister Arseny Yatseniuk called for national unity, urged people to refrain from violence and said they would support constitutional change, decentralising more power to local councils, including over their official language - a key demand of Russian-speakers.

Meanwhile, Russia on Friday said it was disappointed with the United States' assessment of an international deal to defuse the crisis in Ukraine, saying the threat of new sanctions against Moscow by Washington was "completely unacceptable."

The Russian Foreign Ministry, in a statement, accused US officials of seeking to "whitewash" what it said was the use of force by Kiev's authorities against protesters in Ukraine's Russian-speaking eastern provinces.