'Russia is arming, leading, training, and fighting alongside anti-government forces,' said Tillerson in Vienna during a meeting with foreign ministers from the Organization for Security and Co-operation in Europe (OSCE) countries.
The OSCE ministerial gathering came at a moment of tense relations between Washington and Moscow, where Russia is accused of interfering in the U.S. presidential election and being behind a wave of destructive cyber attacks.Moscow has denied both allegations.
U.S. Secretary of State Rex Tillerson (L) speaks with his Russian counterpart Sergey Lavrov at the OSCE Foreign Ministers meeting in Vienna, Austria, Dec. 7, 2017
'We call for full implementation of the Minsk agreements.We will never accept Russia's occupation and attempted annexation of Crimea.Crimea-related sanctions will remain in place until Russia returns full control of the peninsula to Ukraine,' said Tillerson.
Russian Foreign Minister Sergei Lavrov told foreign ministers at the OSCE meeting that 'all the responsibility is with Ukraine' as far as violence in the east was concerned.
A 2015 deal signed in the Belarusian capital of Minsk called for a cease-fire between Russian-backed separatists fighting Ukrainian forces in eastern Ukraine, a conflict that has left more than 10,000 dead.The deal has been often violated.
The top U.S. diplomat's remarks came after his earlier comments that blamed Russia's activities in Ukraine for 'the single most difficult obstacle' to normalize bilateral relationship between Washington and Moscow.
Tillerson met with Lavrov on the sidelines of OSCE ministerial, where the Russian diplomat refused to answer a shouted question from an American reporter.
The two met in a small conference room, with a large array of Russian camera crews and correspondents said to be present.
An American reporter shouted out to Lavrov: 'What do you think of President Trump's Jerusalem decision?'
Lavrov told her to shout louder, and she did.
'I can't hear you,' said Lavrov while other reporters said the question was clearly audible even at the back of the small room.
In a separate meeting with members of the OSCE monitoring mission in Ukraine, Tillerson said the United States is working with Russia about the U.N. peacekeeping proposal.
'We are. I'm not going to tell you specifically what we get, but we get progress, that's what we get,' said Tillerson, 'We get dialogue, we get cooperation. We don't have it solved. You don't solve it in one meeting.'
While both sides have called for a U.N. peacekeeping force in eastern Ukraine, they disagree on the terms of its deployment.