NATO chief Anders Fogh Rasmussen has opened a meeting of alliance foreign ministers by noting Russia's use of "ambiguous" warfare in Ukraine while saying its door "remains open" to aspirant countries.
The meeting in Brussels is also expected to include discussion of the crisis in Iraq, with U.S. Secretary of State John Kerry in attendance.
In opening remarks, Rasmussen called it a "critical moment for our security."
He said NATO needs to continue to adapt its readiness, as the alliance faces "old threats and new, from Eastern Europe to North Africa and the Middle East."
Rasmussen said NATO has witnessed Russia using a "new, different type of warfare" against Ukraine and has seen "no signs" Russia is respecting its international commitments.
He said NATO will discuss how to improve its understanding of "ambiguous threats and how we deal with them in the longer term."
He also said NATO will open "intensified" talks with Montenegro and assess by the end of 2015 whether to invite Podgorica to join.
He said a substantive package is being developed for Georgia to help it move closer to the alliance but made no mention of membership.
As for Iraq, Kerry met late on June 24 with EU foreign policy chief Catherine Ashton and other European allies and discussed what a spokeswoman called the "grave security situation in Iraq."
Kerry flew in from Iraq, where he met on June 24 with officials in the northern autonomous Kurdish region following talks a day earlier in Baghdad with Prime Minister Nuri al-Maliki.
The Brussels meeting is the final top-level gathering before a summit of NATO leaders scheduled for September in Wales.
With reporting by AP, AFP, and Reuters