Ukraine Prime Minister Arseniy Yatsenyuk (L) meets with United Nations Secretary-General Ban Ki-moon at their meeting at UN Headquarters in New York, March 13, 2014. [Photo/Agencies]

Tensions in Ukraine escalated Thursday as a pro-Kiev protester was reportedly killed in the southeastern city of Donetsk, marking the first death since tensions moved from Kiev to Crimea in the southeast.

UN Secretary-General Ban Ki-moon on Thursday voiced his "increasing concern" about developments in Ukraine, underlining the importance to find a peaceful solution to the crisis.

"The secretary-general expressed his increasing concern about developments in Ukraine, including his particular alarm at the rapidly evolving crisis in Crimea and reports of rising tensions in eastern Ukraine," said Ban's spokesman Andrii Deshchytsia.

"The secretary-general underscored that he had intensified diplomatic efforts over the past days and weeks but that all parties must de-escalate the situation, tame the rhetoric and find a peaceful solution to the crisis," the spokesman said.

"He also stressed the need to govern Ukraine in a spirit of inclusiveness and to promote a country of unity and opportunity for all, including minorities," said the spokesman.

Meanwhile, US Secretary of State John Kerry on Thursday threatened that his country and the European Union (EU) would take "a very serious series of steps" against Russia in response to Crimea's vote on Sunday to break away from Ukraine.

"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 the options that are available to us," Kerry told a Senate hearing.

"Now our choice is not to be put in the position of having to do that," he added. "Our choice is to have a respect for the sovereignty and independence and integrity of the country of Ukraine. Our hope is to have Russia join in respecting international law."

Also on Thursday, German Foreign Minister Frank-Walter Steinmeier said that the EU will impose further sanctions if there is no sign of de-escalation from Russia over a referendum in Crimea.

In another development, the Russian Defense Ministry said Thursday that Moscow has deployed six SU-27 fighters and three military transport jets in Belarus, apparently in reaction to growing NATO activities near the Belarusian border.

Russia's confirmation came hours after Belarus released a statement on the deployment.

"In the event of further accumulation of troops formations in the states adjoining Belarus, it will take adequate measures of response on its territory," the Belarusian Defense Ministry said.

"If NATO or someone is making any movements over there in Poland, we have a plan for such a contingency, and we would have to implement it ... I suggested to Russia: let's implement it," Belarusian President Alexander Lukashenko was quoted as saying by the Interfax news agency.

Liu Jieyi, China's UN envoy, called for restraint.

"It is the first priority for all parties concerned to exercise calm and restraint and prevent further escalation of the tension," Liu said.