Russia-Ukraine updates: US to ban Russian carriers from its airspace

Russia claimed Thursday to have neutralized some of Ukraine's military infrastructure amid an attack on the country.

"The military infrastructure of air bases of Ukraine's Armed Forces has been rendered inoperable," the Russian Ministry of Defense said in a statement. "Air defense systems of the Ukrainian Armed Forces have been suppressed."

The Russian defense ministry further alleged that Ukrainian forces on the border "are offering no resistance to Russian units."

Meanwhile, a statement from the Ukrainian Ministry of Defense claimed that five Russian planes and a helicopter had been shot down.

"Reports of foreign media on a Russian aircraft allegedly downed on the Ukrainian territory have nothing to do with the reality," the Russian defense ministry said Thursday.

ABC News could not independently verify the claims on either side.

-ABC News' Tanya Stukalova

In a new security alert, the State Department said it has suspended its consular operations in Lviv in western Ukraine amid “reports of Russian attacks on targets in a number of major Ukrainian cities, including Kyiv, Kharkiv, Odesa, Mariupol and others.”

The U.S. Embassy in Kyiv previously suspended operations on Feb. 12.

“The U.S. government will not be able to evacuate U.S. citizens from Ukraine,” the warning stated.

The State Department advised U.S. citizens to shelter in place and issued instructions on actions to take if a loud explosion is heard or if sirens are activated.

“Further Russian military action can occur at any time without warning. U.S. citizens throughout Ukraine are strongly encouraged to remain vigilant and take appropriate steps to increase their security awareness,” the warning read. “Know the location of your closest shelter or protected space. In the event of mortar and/or rocket fire, follow the instructions from local authorities and seek shelter immediately. If you feel that your current location is no longer safe, you should carefully assess the potential risks involved in moving to a different location.”

-ABC News’ Conor Finnegan

Ukraine’s border service said Russian and Belarusian troops are now attacking from Belarus.

Ukraine’s border came under attack from artillery, tanks and small arms around 5 a.m. local time from Russian troops “with the support of Belarus,” the border service said in a statement.

The attack is happening along much of Ukraine’s northeast border, including the Chernigiv and Zhitomirsky regions that are directly north of Kyiv.

There are reports of casualties.

Ukrainian President Volodymyr Zelenskyy declared martial law in the country Thursday, saying Russia has launched an "unjustified, false and cynical invasion."

"There are strikes on military and other important defense facilities, attacked border units, the situation in the Donbas has degraded," Zelenskyy said in a statement Thursday morning. "The Armed Forces, all special and law enforcement agencies of the state are on alert. The National Security and Defense Council is working in an emergency mode."

"Civilian citizens of Ukraine should stay at home," he added. "Warn your loved ones about what is happening. Take care of those who need help. All thoughts and prayers with our soldiers."

Martial law allows military authorities to temporarily take over government functions, generally during a time of emergency.

The announcement came as reports of explosions and air raid sirens in cities across Ukraine rolled in and as Russian-controlled separatists, in a breakaway region of eastern Ukraine known as Donbas, say they have launched a full-scale offensive to retake what they claim is their territory there.