NATO chief Mark Rutte warned in Warsaw on Wednesday that the Western defence alliance would respond with a "devastating" blow to any attack by Russia on Poland or another ally.
Poland and the nearby Baltic states -- all NATO members -- have ramped up defence spending and training since Russian troops invaded Ukraine in 2022, voicing fears that Moscow could target them next.
Under NATO's Article Five, an attack on one member is considered an attack on all.
"If anyone were to miscalculate and think they can get away with an attack on Poland or on any other ally, they will be met with the full force of this fierce alliance," Rutte told reporters on a visit to the Polish capital.
"Our reaction will be devastating. This has to be very clear to (Russian President) Vladimir Vladimirovich Putin and anyone else who wants to attack us," he said alongside Polish Prime Minister Donald Tusk.
Rutte's visit comes as the United States pushes for a quick end to the Ukraine war, negotiations in which European partners have been sidelined.
"Let's be honest, our influence on these negotiations is limited, to put it mildly," Tusk said.
"So as NATO, as Poland, we need to be all the more ready for scenarios that we haven't entirely envisaged," he added.
"When the geopolitical situation is changing before our eyes, the... confirmation that NATO is obliged to defend Poland in every critical situation, is very important to us."