WASHINGTON — The Pentagon is planning a NATO working-group meeting in Poland next month without several of the alliance’s prominent members, including the United Kingdom, France and the Baltic countries, raising concern among diplomats that Washington is dividing allies into favored and unfavored camps. Pentagon policy chief Elbridge Colby has led the effort, which is intended to discuss NATO’s future. The six countries invited so far are Norway, Sweden, Finland, Poland, Germany and the Netherlands, according to diplomats familiar with the planning. The group, described as the Bergen Group format, has already held two similar meetings. Diplomats from excluded countries say they have received little explanation of the criteria for participation. One official from an excluded nation said its government was trying to understand the meeting’s purpose while continuing to meet U.S. expectations. The exclusions are drawing particular notice because Britain and France are major European defense powers, while the Baltic states are central to NATO’s security posture along its eastern flank. The alliance is confronting increased Russian aggression and pressure on weapons stockpiles, adding to concerns that internal divisions could weaken coordination. A diplomat from an invited country said the smaller meetings are intended to develop proposals for discussion across NATO, not to split the alliance. The diplomat acknowledged that some excluded members are frustrated, including countries that have met or exceeded defense-spending targets promoted by the Trump administration. Colby said after a meeting in Bergen, Norway, in June that the invited allies were moving quickly toward spending commitments agreed at the NATO summit in The Hague, had the capacity to contribute and were ready for practical work. At a Washington conference this month, he said the aim was to bring the whole alliance along and build European military capacity. The effort comes amid repeated criticism of NATO from President Donald Trump and senior administration officials. Trump has complained that the United States spends heavily to defend allies that, in his view, do not reciprocate. He and his advisers have also criticized members over defense spending and their decisions not to support U.S. policy, including during the war with Iran. Defense Secretary Pete Hegseth has called for rewarding “model allies” and warned that countries that do not support U.S. interests could face consequences. The White House previously considered a system for distinguishing allies based on whether they followed Trump’s priorities, according to reporting published in April. Another person familiar with the planning said Denmark could be invited in the future, following an agreement with Washington to expand the U.S. military presence in Greenland. Even some governments included in the working group have questioned why other major European defense powers were left out. One diplomat warned that sorting NATO members into “good” and “bad” allies could endanger the United States and pose an existential challenge for Europe.