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, RFE/RL reports.

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."

The Brussels meeting is the final top-level gathering before a summit of NATO leaders scheduled for September in Wales.