UN Secretary-General Ban Ki-moon is heading to Russia and Ukraine amid the escalating crisis over Crimea.
Ban's office said he will meet with Russian President Vladimir Putin on March 20, before traveling to Kyiv on March 21 for talks with top Ukrainian officials to push for a peaceful resolution.
Meanwhile, British Prime Minister David Cameron on March 19 said Russia could face permanent expulsion from the Group of Eight (G8) industrialized countries if it took further action against Ukraine after annexing Crimea.
And U.S. Vice President Joe Biden, on a visit to Lithuania, said Russia was on a "dark path" to isolation.
Speaking alongside Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins, Biden promised that NATO would respond "to any aggression" against its member states.
He said U.S. President Barack Obama would want to use an upcoming NATO summit in Wales to ensure members can meet Article 5 collective-defense commitments.
Grybauskaite called Russia's actions in Crimea a "direct threat to our regional security" and said all measures must be taken to protect against threats.
Biden's visit to Vilnius was part of a two-day regional trip to reassure allies following Moscow's moves. Biden was in Warsaw on March 18, where he met with Polish Prime Minister Donald Tusk and Estonian President Toomas Hendrik Ilves.
In related news, German Foreign Minister Frank-Walter Steinmeier called for quick agreement on a mandate for an OSCE observer mission to Ukraine, which the West says should also include Crimea.
The United States and the European Union have denounced Crimea's annexation as a "land grab."
Meanwhile, the Russian Foreign Ministry has accused the United States and the EU of breaking a 1994 security agreement guaranteeing Ukraine's sovereignty by "pandering" to what it termed a "coup" in Kyiv.
The UN Security Council is expected to hold an emergency session on the crisis later on March 19.
Based on reporting by AFP and Reuters