A Ukrainian skeleton star defied demands for politics to be kept out of the Winter Olympics by holding up a "NO WAR IN UKRAINE" sign to television cameras on Friday.

Vladyslav Heraskevych, 23, is competing at his second Games and was in 17th place after the third run of four in the men's event.

Immediately after getting off his sled, Heraskevych walked towards the cameras and revealed his small banner, with its message set against a background of the Ukraine national flag.

It remains to be seen what action, if any, the International Olympic Committee (IOC) takes against Heraskevych.

His political statement comes at a time when tens of thousands of Russian troops are said to have gathered on the Ukrainian border, amid concern of conflict and a possible invasion.

NATO secretary general Jens Stoltenberg said on Thursday: "This is a dangerous moment for European security. The number of Russian forces is going up. The warning time for a possible attack is going down."

The IOC is clear on its apolitical standing, however, and rule 50.2 states: "No kind of demonstration or political, religious or racial propaganda is permitted in any Olympic sites, venues or other areas."

Those who breach the rule "may be subject to the IOC's disciplinary proceedings", the IOC said.

Russian president Vladimir Putin was among the world leaders who attended the Beijing 2022 opening ceremony.