Russian President Vladimir Putin on Thursday announced an operation in Ukraine in a surprise televised address. Putin said the action came in response to threats that he claimed came from Ukraine.

"I have made the decision of a military operation," he said in a statement shortly before 6 a.m. (0300 GMT). Putin called on Ukraine's military to "lay down its arms." Ukraine's military to "lay down its arms."

He said Russia did not have a goal to occupy Ukraine, and warned other countries that any attempt to interfere with the Russian action would lead to "consequences they have never seen.''

UN chief tells Putin not to attack Ukraine

UN Secretary-General Antonio Guterres urged Putin not to order troops to attack Ukraine during an emergency security council meeting in New York. "The day was full of rumors and indications that an offensive... was imminent," Guterres said.

"If indeed an operation is being prepared, I have only one thing to say from the bottom of my heart, Guterres added. "President Putin, stop your troops from attacking Ukraine, give peace a chance."

The US envoy to the security council said during the meeting that the US and Ukraine believe that a "full-scale, further invasion into Ukraine by Russia is imminent."

​​Airspace deemed hazardous

Airlines should stop flying over Ukraine due to the risk of an unintended shootdown, a conflict zone monitor said. The monitor also mentioned the possibility of cyberattacks targeting air traffic control.

Safe Airspace said it had increased its risk level to "do not fly." The organization was set up to provide safety and conflict zone information for airlines after Malaysia Airlines flight MH17 was shot down over eastern Ukraine in 2014.

"Regardless of the actual movements of Russian forces into Ukraine, the level of tension and uncertainty in Ukraine is now extreme," Safe Airspace said.

"This itself gives rise to significant risk to civil aviation."

Ukraine said that flights of civilian aircraft in its airspace are "restricted due to potential hazard for civil aviation," according to a notice to airmen issued at 0156 GMT on Thursday.

The notice is due to expire 2359 GMT on Thursday unless extended.

Airports in the Ukraine's central-eastern cities of Dnipro, Kharkiv and Zaporizhzhia are closed to traffic until Thursday morning, also according to notices to airmen.