Russian forces broke into the Kyiv region, as Russia's Federal Security Service (FSB) claimed Ukrainian border guards have left all facilities on the Russia-Ukraine border.
An AFP reporter in the northern part of Ukraine's capital also saw several low-flying helicopters flying toward the city, amid reports that an airfield was under attack.
The Ukrainian official said they feared that Russian forces could be airdropped into the country and then try to penetrate the government district in Kyiv.
The FSB said Ukrainian border guards had abandoned all facilities at the Russian-Ukrainian border, the Interfax news agency reported.
Ukraine's armed forces find themselves vastly outnumbered and outmatched in firepower by their Russian opponents, despite growing military assistance for Kyiv's troops by the West.
Russian President Vladimir Putin, whose country had massed some 150,000 troops on the border in recent months, on Thursday ordered military operations against Ukraine.
There are also, according to Western estimates, some 30,000 Russian troops deployed in Belarus, ostensibly for exercises, who could also attack Ukraine from the north.
And Russia has massed naval forces in the Black Sea and closed to navigation the Sea of Azov between Russia and Ukraine.
According to the London-based International Institute for Strategic Studies (IISS), Russia's armed forces amount to 900,000 troops, plus some two million reserves and more than half a million other forces.
Ukraine's forces meanwhile amount to barely more than the number of troops Russia had amassed around its borders, with a standing army of 145,000, 45,000 in the air force and 11,000 in the navy, according to the IISS.
It has some 100,000 other forces and 900,000 reserve soldiers.
Analysts also note that the gulf in firepower is even wider in terms of military hardware, with Russia's almost 16,000 armoured fighting vehicles -- including tanks -- dwarfing the Ukrainian fleet of 3,300.
Artillery numbers show a similar difference, while the Ukrainian air force is a tenth the size of its Russian counterpart.
"The military balance of power is totally overwhelming" in favour of Moscow, said Francois Heisbourg, special advisor to the Foundation for Strategic Research (FRS) in Paris.
