Ukrainian Prime Minister Arseniy Yatsenyuk has told the UN Security Council that his country has faced a military aggression that has "no reason and no grounds," but he said it was still possible to peacefully resolve the crisis with Russia.
"We urge [the] Russian Federation to pull back its military forces deployed in Crimea to barracks and to start real talks and negotiations in order to tackle this conflict," Yatsenyuk said.
Yatsenyuk said that Russia's "military aggression" in Crimea is undermining global security and nuclear nonproliferation.
"The way our Russian neighbors -- and I believe that if we start real talks with Russia, they could be real partners -- acted, undermines the entire global security and nuclear non-proliferation program," Yatsenyuk said.
"As after these actions, it will be very difficult to convince anyone in the globe not to have nuclear weapons."
Turning to Russian Ambassador Vitaly Churkin, Yatsenyuk -- who until then had been delivering his address in English -- asked Churkin in Russian whether Russia really wanted war.
Yatsenyuk's address came as tensions escalate ahead of a referendum in Crimea, now occupied by Russian forces, on whether to join Russia.
In the eastern city of Donetsk, a man was killed after pro-European demonstrators were attacked by pro-Russian supporters.
A further dozen were injured and taken to hospital.
Russian Ambassador Vitaly Churkin said Russia did not want war in Ukraine, while urging the international community to respect Crimea's right to self-determination.
U.S. Ambassador Samantha Power reiterated the U.S. position that the March 16 referendum called by Crimea's pro-Russian leadership is illegal under international law. She said "None of us can afford to leave any stone unturned, but Russia has to want a diplomatic solution."
Earlier, U.S. Secretary of State John Kerry warned that the United States and the EU would unveil "a series of very serious steps" on March 17 should Russia accept and act on the widely expected vote in favor of joining Russia in the referendum.
"There will be a response of some kind to the referendum itself and in addition If there is no sign of any capacity to be able to move forward and resolve this issue there will be very serious series of steps on Monday in Europe and here with respect to the options that are available to us," Kerry said.
(WATCH: Merkel Warns Russia Of 'Massive' Damage)
In Berlin, German Chancellor Angela Merkel said the EU is prepared to impose targeted sanctions on Russian and Ukrainian officials if Moscow refuses to begin "negotiations that achieve results."
"If Russia continues to further destabilize the situation in Ukraine, including the east of Ukraine where we see a worrying development, the EU leaders have agreed to take further action," Merkel said.
"Measures could concern the economic cooperation with Russia in various aspects. To put it clearly: No one wishes to take those measures, but we would all be ready for it and determined should they become necessary."
Secretary of State Kerry plans to meet with his Russian counterpart, Sergei Lavrov, in London on March 14 in a last-ditch effort for a diplomatic solution.
A State Department official said that in a phone call with Lavrov on March 13, Kerry reiterated the United States’ concern about the upcoming referendum, and made clear there will be costs if Russia continues to take escalatory steps.
With reporting by Reuters, AP, AFP, dpa, and UNIAN