Nato foreign ministers are due to discuss ways to help Ukraine and reassure allies in Eastern Europe, at a meeting in Brussels.
It is the first time ministers from the 28 member states of the alliance have convened since Russia's takeover of Crimea caused a diplomatic crisis.
Nato has also bolstered air drills to be held over the Baltic states.
On Monday, Russia ordered a partial withdrawal of its troops near the eastern border of Ukraine.
Moscow is believed to have massed tens of thousands of soldiers there in recent days, causing alarm in Kiev and the West.
Russian President Vladimir Putin told German Chancellor Angela Merkel he had ordered the pullback, according to the German government.
Continue reading the main storyMoscow has broken with a pattern of behaviour that has characterised diplomacy in Europe since the end of the Cold War”End Quote
Jonathan Marcus BBC diplomatic correspondent Meanwhile, Russian energy firm Gazprom is increasing the price it charges Ukraine for gas from Tuesday. Gazprom's chief executive Alexei Miller said the price of Russian gas for Ukraine had gone up to $385.5 (£231) per 1,000 cubic metres in the second quarter of 2014 from the previous rate of $268.5.
Mr Miller added that Ukraine's unpaid gas bills to Russia stood at $1.7bn.
In other developments on Tuesday:
Ukraine's parliament ordered security services to disarm all "illegal armed groups", following Monday night's shooting in Kiev that involved a member of the radical Right Sector groupRussia's upper house of parliament voted to pull out of a treaty with Ukraine on the Black Sea Fleet's presence in CrimeaTensions between the Kremlin and the West rose after the overthrow of pro-Kremlin Ukrainian President Viktor Yanukovych in February, following months of street protests.
Russia's subsequent decision to annex Crimea in March triggered a crisis in relations.
The US and EU have imposed sanctions on members of President Putin's inner circle and other officials. Russia has retaliated with its own sanctions on US politicians.
Rattled nerves
Nato foreign ministers are expected to discuss the formal suspension of co-operation with Moscow at the Brussels meeting.
Russia has said the troops deployed along its border with Ukraine are taking part in military exercises 
Ukrainian forces have been leaving Crimea after Russia's military takeover of the peninsula 
The US Air Force has sent 10 F-15 jets to help Nato boost its military presence in the Baltic states 
Ukraine's special forces reportedly seized weapons from members of the radical Right Sector group In a statement, the alliance said ministers would speak to acting Ukrainian Foreign Minister Andriy Deshchytsia about ways to support Ukraine with its defence reforms.
They are also expected to look at options including situating permanent military bases in the Baltic states to reassure members in Eastern Europe.
Russia's actions in Ukraine have rattled nerves in Estonia, Latvia and Lithuania, which were part of the Soviet Union during the Cold War.
Nato jets will take part in air patrols in the region later in a routine exercise that analysts say has taken on added significance due to the crisis.
Several Nato countries, including the UK, US and France, have offered additional warplanes.
'Crude violation'Earlier, Ukraine condemned a visit to Crimea by Russia's Prime Minister Dmitry Medvedev and a delegation of government ministers.
A foreign ministry spokesman in Kiev said the highest-level trip to the Black Sea peninsula by officials from Moscow since its annexation by Russia was a "crude violation" of international rules.
Crimeans voted to leave Ukraine for Russia on 16 March, in a Moscow-backed referendum that was later condemned as illegal by the UN General Assembly.
Mr Medvedev announced that he would make Crimea a special economic zone, with tax breaks and reduced bureaucracy to attract investors.
He also vowed to quickly boost salaries and pensions, and to improve education, healthcare and local infrastructure.
However, a number of local residents have complained they are yet to see the promised reforms.