British Foreign Secretary William Hague has called the growing crisis in Ukraine the "biggest in Europe in the 21st Century" as 6,000 Russian troops took control of the Crimean Peninsula in Ukraine on Monday.
"Clearly we are very concerned about any possibility of a further move by Russia in other parts of Ukraine but that does not mean the position in the Crimea is stable," Hague told BBC.
"This is a very tense situation and dangerous situation that Russia's intervention has now produced."
Fox News reported on Monday that 6,000 Russian airborne and naval forces have now taken "complete operational control of the Crimean peninsula" without suffering any causalities, according to a senior Obama administration official, despite repeated warnings and concern from western powers such as the U.S. and members of the European Union.
Moscow has argued that it is defending human rights and citizens in Ukraine loyal to Russia, with Russian Foreign Minister Sergey Lavrov criticizing those speaking out against President Vladimir Putin's government at a U.N. human rights meeting in Geneva, Switzerland.
"Those who attempt to interpret the situation as an act of aggression and threaten us with sanctions and boycotts – these are the very same partners of ours who consistently have encouraged political forces close to them to deliver an ultimatum and refuse dialogue, to ignore the concerns of southern and eastern regions of Ukraine, which has ultimately polarized Ukrainian society," Lavrov said, according to CNN, claiming that Ukraine's new government, which ousted former President Viktor Yanukovych last week, is attacking minorities and violating human rights.
Ukraine Interim Prime Minister Arseniy Yatsenyuk has promised, however, that his country will not give up the Crimean region.
"Nobody will give Crimea away. ... There are no grounds for the use of force against civilians and Ukrainians, and for the entry of the Russian military contingent," Yatsenyuk said at a news conference. "Russia never had any grounds and never will."
On Sunday, U.S. president Barack Obama spoke with Prime Minister David Cameron of the United Kingdom, President Bronislaw Komorowski of Poland, and Chancellor Angela Merkel of Germany about the crisis in Ukraine.
"The leaders expressed their grave concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law and a threat to international peace and security," the White House said in a readout of the conversation. "The leaders stressed that dialogue between Ukraine and Russia should start immediately, with international facilitation as appropriate."
Obama and the other leaders also "affirmed the importance of unity within the international community in support of international law and their support for the Government of Ukraine, including its territorial integrity and its efforts to move forward with elections in May so that the Ukrainian people can continue to determine their own future in this historic hour" and vowed to continue coordinating closely.
Faith leaders have also spoken out about the need of the international community "to support any initiative for dialogue and harmony," with Pope Francis declaring in his Sunday liturgy that Christians need to reach out to those in need.
The Roman Catholic Church leader further expressed hopes that all parts of the country "will endeavor to overcome misunderstandings and build together the future of the nation."