Ukraine's Security Council chief has accused Russia of planning to invade after it claimed that tens of thousands of troops have amassed on the country’s border.
Andriy Parubiy, Secretary of Ukraine's National Security and Defense Council, said during a briefing in Kiev on Wednesday's, that Russia had deployed more than 80,000 troops, up to 270 tanks, and 140 combat planes close to the border - a move which he said signified the “threat of a full-scale invasion from various directions.”
The comments come after Russian forces secured control over Ukraine's Crimean Peninsula late last month, when President Vladimir Putin said Moscow is prepared to “use all means” to protect Russian speakers in eastern Ukraine.
The Kremlin-controlled parliament has since granted Putin permission to use the Russian military in Ukraine.
However, Deputy Defense Minister Anatoly Antonov denied the buildup on the Russia-Ukraine border and said Moscow had accepted a request on Tuesday which would allow Ukraine to conduct a surveillance flight over Russian territory to check for a threat.
The request was made under the Open Skies treaty that was signed in 1992, and enables Russia and Ukraine to carry out surveillance flights over each other’s territories.
In pictures: Ukraine crisis
In pictures: Ukraine crisis
1/12 Ukraine crisis
People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea.
2/12 Ukraine crisis
In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea.
3/12 Ukraine crisis
Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea.
4/12 Ukraine crisis
Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol.
5/12 Ukraine crisis
Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II.
6/12 Ukraine crisis
A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise.
7/12 Ukraine crisis
An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol,
8/12 Ukraine crisis
9/12 Ukraine crisis
Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th.
10/12 Ukraine crisis
An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th.
11/12 Ukraine crisis
Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya.
12/12 Ukraine crisis
Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting.
But Antonov added that this was the first request of this kind from Kiev, and that Moscow was “under no obligation” to allow it immediately.
"We have decided to allow such a flight. We hope that our neighbors are assured that there is no military activity that threatens them on the border," he said.
Ukrainian authorities have denied 3,700 Russian citizens permission to enter Ukraine, he added, because they were suspected of being involved in extremism and sabotage.
Crimea, where Russia maintains its Black Sea Fleet base, became the focus of tensions in Ukraine after former President Viktor Yanukovych fled last month following months of pro-EU protests.
Elsewhere, Ukraine's interim Prime Minister Arseniy Yatsenyuk planned to meet with President Barack Obama in Washington on Wednesday, after he called on Western countries to defend Ukraine against Russia.
He accused Moscow of being "armed to the teeth" and in possession of nuclear weapons.
On Wednesday, Nato deployed two surveillance aircraft to monitor Ukraine's air space and Black Sea ship movements as Russia consolidated its military buildup in Crimea.
Meanwhile, Pentagon spokesman, Army Colonel Steve Warren, said that the US is sending 12 F-16 fighter jets to Poland to augment the air force detachment there.
There is no scheduled departure date for the fighter jets and they will be there "until further notice," he added.
The deployment comes after the Pentagon sent six F-15 fighter jets to Lithuania to bolster air patrols over the Baltics, adding to the four such planes that previously had been there for the mission.
As Washington considers imposing new sanctions on Russia, US Secretary of State John Kerry will hold with his Russian counterpart, Foreign Minister Sergey Lavrov, in London on Friday in a bid to defuse the crisis over Ukraine.
Additional reporting by AP