Russia will come under heightened diplomatic pressure as the UN Security Council and European leaders hold emergency talks on Ukraine, after the seizure of Crimea created the worst East-West crisis since the Cold War.
The European Union summit in Brussels starts on Thursday, when leaders will meet with Ukraine's Prime Minister Arseniy Yatsenyuk, who took over after the overthrow of president Viktor Yanukovych in February following three months of deadly protests.
The EU on Thursday froze assets held by 18 Ukrainians accused of embezzlement, including ousted Moscow-backed Mr Yanukovych and his son Oleksandr.
As the EU discusses the crisis, 40 unarmed military personnel are expected in Crimea in a mission by the Organisation for Security and Co-operation in Europe to try to defuse tensions in the region.
Later, the 15-member United Nations Security Council will hold closed-door talks in New York - its fourth consultations on the subject since 28 February.
During an acrimonious round of emergency talks on Monday, Russia told other council members that Mr Yanukovych had asked Moscow to send troops to re-establish law and order in his country.
As a permanent member of the council, Russia holds veto power and can block the body's draft resolutions.
UN special envoy to Crimea Robert Serry was forced to cut short a visit when confronted by unidentified gunmen on Wednesday, but was expected to return to Kiev soon.
Pro-Russian forces entered and took over parts of a Ukrainian missile base on Wednesday, while a Ukrainian court ordered the arrest of Crimea's newly installed pro-Russian prime minister Sergiy Aksyonov for separatism.
Violent protests have also broken out in cities in mainly Russian-speaking eastern Ukraine, where several regional government buildings have been taken over by pro-Russian militants who have clashed with police.
However, US Secretary of State John Kerry said he was hopeful of a "de-escalation" in the stand-off on the eve of the EU summit after meeting Russian counterpart Sergei Lavrov, who pressed for early elections in Kiev.
Mr Yanukovych fled to Russia following a bloodbath in Kiev in which scores of protesters and police officers were killed and, although he insists he is the legitimate president, Moscow has signalled his time is over.
Western leaders have increased diplomatic pressure on Russia, with some threatening to boycott the G8 summit in Russia in June and NATO saying it will review a series of accords with Moscow.