A spat over grain has caused frustration on both sides this week.
More of the mercenaries are gathering in Belarus, which borders several Nato members.
It comes after Poland sent troops to the border amid worries about Belarus.
“I just want to go home. It’s my town, it’s my flat, it’s my friends.” Hanna Semenenko, who now lives in Oxford, has just returned from a trip to Kyiv.
The Spice Girls singer had been due to perform in the city of Zakopane on Saturday.
Tensions rose after a missile killed two people on the Ukraine-Poland border on Tuesday night.
Nato's Article 5 pledge of mutual defence explained as fears grow over escalating tensions in the war in Ukraine.
Polish media reported that two people died on Tuesday afternoon after a projectile struck an area where grain was drying in Przewodów.
Another day, another politician who thinks we're baby-making machines.