Russia is preparing for a protracted war, so NATO should prepare for the long term and support Ukraine for as long as necessary, the alliance's deputy secretary general Mircea Geoană said, AP reports.

Speaking at the opening of the military chiefs' meeting here, Geoană said NATO nations should invest more in defense, increase military-industrial production and use new technology to prepare for future wars.

U.S. Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff, met secretly Tuesday with Ukrainian Commander-in-Chief Gen. Valery Zaluzhny in southeastern Poland, near the border with Ukraine.

Milley's meeting with Zaluzhny was arranged quickly when it became clear Monday that he would not be able to attend NATO meetings in person. He is expected to take part in a videoconference on Thursday.