Ukraine's interim president Oleksandr Turchynov says an anti-terrorist operation involving the army will begin in eastern Ukraine, unless rebels lay down their arms.
Kiev's deadline for pro-Russian forces to withdraw from towns they are occupying passed on Monday with no sign of compliance.
Mr Turchynov says the Donbass region would soon be stabilised. However, he also said his leadership is "not against" a referendum being held on the type of state Ukraine should be and is sure it would confirm the wish of the majority for a united, independent country, Reuters reports.
The holding of a referendum has been one of the demands of separatist rebels in eastern Ukraine.
On Sunday night, Russia and Ukraine clashed at an emergency session of the United Nations, accusing each other of terrorism.
Russian ambassador Vitaly Churkin said Ukraine was waging war on its own people in the east of the country. Ukrainian representative Yuriv Sergeyev reacted by alleging that Moscow was out to destabilise Ukraine and bring it down.
"During the last month and a half in the eastern regions of Ukraine, agents of the Russian special service embarked on the large scale operations to seize power, destabilise the situation, threatening the lives of citizens of Ukraine for the separation of those regions of our country."
Russia urged authorities in Kiev not to use force against pro-Russian protesters in eastern Ukraine and called for them to "start a genuine dialogue", the BBC reports.
Pro-Russian forces have targeted several cities in eastern Ukraine in the past week. Ukraine's government has promised military action against them if they do not relinquish control of government buildings by the early hours of Monday.
Mr Churkin said that the opinions and interests of protesters in eastern Ukraine had not been taken into consideration.
European Union foreign ministers are due to meet later on Monday in Luxembourg to discuss the crisis.
The US said Russia was "spreading fiction" and that the events of the past week mirrored those preceding the annexation of Crimea.
"We know who is behind this," said Samantha Power, the United States' permanent representative to the UN.
Ms Power said the US would still take part in talks on the crisis planned for Geneva on Thursday. However, she said if the talks were to succeed, Russia would have to explain the build-up of 40,000 troops on Ukraine's eastern border.
Sunday's meeting was the 10th time the Security Council has convened to discuss the Ukraine crisis. The meeting was called for by Russia, which has strongly criticised Kiev's plan to use its armed forces in eastern Ukraine.
Acting Ukrainian President Oleksandr Turchynov earlier said he would not allow a repetition of what happened in Crimea which was annexed by Russia in March.
Eastern Ukraine has a large Russian-speaking population and has seen a series of protests since the ousting of Ukraine's pro-Russian President Viktor Yanukovych in February.