Russia is targeting Ukraine's military infrastructure, air defense, and air forces with high-precision weapons and is not attacking Ukrainian cities, Russia's RIA news agency cited the Russian Defense Ministry as saying on Thursday.

Kyiv said Moscow has launched a full-scale attack on Ukraine, with military command centers in a number of cities hit by missiles.

An adviser to the Ukraininan President said the country is being hit by a second wave of missile strikes.

Authorities in Kyiv say the first wave, launched shortly after Russian President Vladimir Putin ordered a military operation on Thursday morning, hit military command centres and other buildings in several Ukrainian cities.

Ukrainian President Volodymyr Zelenskyy Thursday announced martial law across the country as Russian troops entered the border and hit targets around Kyiv.

In a video posted on his Telegram channel, Zelenskyy said Russia launched strikes against Ukrainian military facilities and border guard units.

"Explosions were heard in many cities of Ukraine. We are declaring martial law across the country," he said.