Poland is a voice for nations on the EU's eastern flank that face constant hybrid threats from Russia, including influence campaigns and disruptive drones.
Europe has been reluctant to leave the talks solely in the hands of a US administration that has often seemed to lean toward Moscow's views.
Costa said it was essential to have a team and to speak with only one voice."And for this it's essential to have a team and to speak with only one voice,"Costa said.
Russia, for its part, does not want European countries involved in direct negotiations. Moscow complains that they are financing and supplying Ukraine's military effort.
Kallas said,"I know that the Russians don't want me, but, you know, do we have a say, who is representing Russia?".
Ukraine is eager for support from its European allies as the question of who sits at the table is settled.
The question of who represents Europe is an old one. Earlier this year, Costa faced criticism within the EU after his office set up a hush-hush back-channel with Moscow.