NATO defence ministers wrapped up two-day talks in Brussels on Thursday, focusing on updates to the NATO command structure, increases in fair burden sharing, and NATO-EU cooperation.
"Secretary Mattis thanked the minister for hosting USA forces and expressed appreciation for Poland's continuing contributions to regional and global security", she said.
Krstičević and Montenegrin Defence Minister Predrag Bošković agreed to enhance the cooperation between the two countries' armed forces, underlining the importance of the joint training of troops for North Atlantic Treaty Organisation missions.
They also made a decision to establish a new support command to improve the movement of troops in Europe and boost troop rapid response while creating a new operations centre to defend against cyber attacks in Mons, Belgium.
North Atlantic Treaty Organisation ministers met Wednesday with European Union foreign policy chief Federica Mogherini to discuss their concerns over duplication after Brussels agreed in December to develop new military equipment and improve cooperation and decision-making.
But on Sunday a senior official working with Mattis said Washington had concerns that some of the proposed initiatives risked "pulling resources or capabilities away from NATO".
"Certainly we do not want this to be a protectionist vehicle for the European Union and we're going to watch carefully, because if that becomes the case then it could splinter the strong security alliance that we have", she told reporters.
The NATO chief reiterated his comments on Turkey's Operation Olive Branch saying Turkey had informed the alliance on the operation.
Mattis is to meet his Turkish counterpart on the sidelines of the meeting on Wednesday and Stoltenberg welcomed the fact that Turkey and the United States were talking directly.
"This is a key part of our collective security", - said Stoltenberg.
"Turkey has some legitimate security concerns".