Ukraine President Petro Poroshenko told British Prime Minister David Cameron that Ukrainian forces have destroyed part of Russian military equipment after it crossed the border into Ukraine during the night, according to the president's website.
More top news
The man, who was in his 30s, has not been identified, while several other people are believed to have suffered serious injuries.
The Olympian was one of several sports stars who opened up to the prince during an event for his mental health charity Heads Together.
There will be sunny spells, with temperatures peaking at 23 Celsius (73F). However, Northern Ireland will experience frequent showers.