Ukraine crisis: Russia begins new military exercises
Russia has begun military exercises, involving more than 8,000 troops, close to the border with Ukraine.
The defence ministry in Moscow confirmed that artillery such as rocket launchers and anti-tank weapons would also be involved in the exercises.
They come at a time of high tension ahead of Crimea's referendum on Sunday on whether to join Russia.
Ukraine PM Arseniy Yatsenyuk has told the UN Security Council his country is a victim of Russian aggression.
He said it was "absolutely and entirely unacceptable, in the 21st Century, to resolve any kind of conflict with tanks artillery and boots on the ground", in reference to Russian troops at key sites in Crimea.
But he also said: "We still believe that we have a chance to resolve this conflict in a peaceful manner."
Meanwhile, reports from the eastern Ukrainian city of Donetsk say at least one person has been killed and several wounded in clashes between hundreds of pro- and anti-Moscow demonstrators.'Serious steps'
Russia confirmed that military exercises had begun in the regions of Rostov, Belgorod and Kursk, which are close to the border of Ukraine, and would continue until the end of March.
"The main aim... is a multi-faceted check of the units' cohesiveness followed by the performance of battle training assignments in unfamiliar terrain and untested firing ranges," the defence ministry said.
In a sign the tension may be spreading, Belarus - a Russian ally - confirmed Moscow had deployed, at its request, extra fighter jets and military transport aircraft after Nato boosted its forces in the neighbouring Baltic countries.
Russian President Vladimir Putin told a meeting of his Security Council that this was an inter-Ukrainian crisis that "arose not through our fault, but we are involved in it, one way or another".
He said he wanted to discuss how to "build relations with our partners and friends in Ukraine and our other partners in Europe and the United States".
Separately, the Organisation for Security and Co-operation in Europe (OSCE) said Russia had now given its support to a possible long-term monitoring mission in Ukraine.
The Swiss ambassador to the OSCE, Thomas Greminger, called it a "positive development", although the BBC's Bethany Bell in Vienna says no final agreement has been reached and details are still being worked out.
US Secretary of State John Kerry is due to meet his Russian counterpart Sergei Lavrov to discuss the crisis in London on Friday, amid fears that Moscow is attempting to annex the mainly ethnic Russian autonomous region of Crimea.
Mr Kerry said the big question was whether Moscow was "prepared to find a way to negotiate with Ukraine... to resolve this in a way which respects their legitimate interests".
"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here with respect to the options that are available to us," he told senators.
UK Foreign Secretary William Hague has warned that EU sanctions will be stepped up if there is no progress in the "next few days".
EU foreign ministers are expected to discuss possible sanctions when they meet in Brussels on Monday.
The European Parliament on Thursday passed a non-binding resolution calling for the EU to consider measures such as an arms embargo and a freeze on the assets of Russians linked to recent events in Ukraine.
German Chancellor Angela Merkel warned Moscow of "massive" political and economic damage if Russia did not change course.
Meanwhile, the 34-member Organisation for Economic Co-operation and Development (OECD) - which promotes good governance and economic policy initiatives - said it was suspending accession talks with Russia.
Both the authorities in Kiev and its Western allies say they will not recognise Sunday's referendum, which they say violates Ukraine's constitution and was hastily arranged with suspected Russian involvement.