KYIV, 24 Feb (CNN-REUTERS) -- Explosions and gunfire were reported in Kyiv and other Ukrainian cities after Russian President Vladimir Putin announced a "special" operation to "demilitarize" the country, a move that could signal the start of heavy fighting between the two nations.

Reports from Western news agencies early on January 24 said that multiple cities were under attack and that ground forces were entering Ukraine from various positions in Russia and Belarus.

Live video from CNN showed explosions in several parts of the capital, Kyiv. Reuters cited a witness as saying explosions had been heard in Kyiv.

Gunfire was heard near the main airport of Boryspil in the Ukrainian capital, Kyiv, soon after Russia announced the operation, Interfax said, citing local media.

There was no immediate report of injuries or deaths, but a spokesman for the Ukrainian Interior Ministry said the long-feared, full-scale invasion had begun.

Initial reports could not all be independently confirmed.

In a nationally televised speech early on January 24, Putin sought to justify the offensive operation by claiming that he has to stop Ukraine from acquiring nuclear weapons, Russian state television reported.

He called on the Ukrainian military to lay down its arms.

U.S. President Joe Biden immediately condemned the operation, calling it an “unprovoked and unjustified' attack on Ukraine and said the world would “hold Russia accountable.”

He said Russia would be held responsible for the “catastrophic loss of life and human suffering” to come.