Explosions could be heard before dawn in Kyiv. Gunfire rattled out near the main airport and sirens blared across the city.

Ukraine's President Zelensky announced that diplomatic relations with Russia had been cut, the country's state media Interfax-Ukraine reported.

Meanwhile, the Ukrainian parliament endorsed a presidential order declaring martial law.

Zelensky also called on all citizens who were ready to defend the country to come forward, saying Kyiv would issue weapons to everyone who wants them.

The Kyiv government sounded air raid sirens more than once in the morning. People poured into supermarkets to buy water and food, and roads toward the west were blocked by massive traffic jams as residents tried to escape the capital.

A resident of Ukraine's second-largest city, Kharkiv, the closest big city to the Russian border, said windows in apartment buildings were shaking from constant blasts. The city was gripped by panic as people tried to flee, said the resident, who asked not to be identified.

Meanwhile, the State Border Guard Service of Ukraine announced in the morning that the border with Belarus had been attacked by troops from Russia and Belarus.

CNN reported tanks from the Belarusian Armed Forces moving from Belarus to Ukraine. Belarusian President Alexander Lukashenko denied the report, saying his country had not participated in Russia's military operation in Ukraine.

Lukashenko added, however, that Belarusian troops could be used in the operation against Ukraine if needed.

Russia claimed that two Ukrainian Air Force Su-24 jets had been shot down, and lots of Ukrainian government soldiers in Lugansk refused to take orders and began leaving the front line.

But the Ukrainian military soon denied that and said they had shot down six Russian fighter jets and helicopters. They said one of their military units in Odessa had been attacked by a Russian airstrike, leaving six dead, seven wounded and 19 missing.

Agencies contributed to this story.