Turkey's procurement of an advanced Russian surface-to-air missile defense system is a "national decision," NATO Secretary General Jens Stoltenberg said Thursday.
Stoltenberg was addressing reporters as the transatlantic alliance marks its 70th anniversary in the U.S. capital.
Turkey's decision to procure the Russian S-400 system has led to significantly strained ties with Washington, which earlier this week suspended delivery of parts and services necessary to Turkey's receipt of the F-35 stealth fighter jet.
U.S. officials have suggested Turkey buy the U.S. Patriot missile system rather than the Russian S-400 system, arguing it is incompatible with NATO systems and exposes the F-35 to possible Russian subterfuge, including covert efforts to obtain critical information on the jet, which could then be relayed to Russia.
The NATO chief said the matter is not on the meeting's agenda, but he expects it to be addressed on the sidelines of Thursday's talks.
"We see that this is an issue which has created disagreement between allies, and NATO provides a platform for allies to address issues like this," Stoltenberg said.
"NATO provides support that augments the air defenses of Turkey already," he added.
In response to the U.S. position the S-400 missile system is incompatible with NATO military equipment, Turkish Foreign Minister Mevlut Cavusoglu said Wednesday the system would be for Turkey's own use.
"It doesn’t have to be integrated into the NATO system, and this is not our aim. It is for our own use," he said. "This system will not see any NATO system, including F-35s, as an enemy.”
Speaking at the formal opening of Thursday's discussions, U.S. Secretary of State Mike Pompeo hailed the mutual defense organization, saying no nation "can match" the collective power of NATO's member states.
He reiterated Washington's calls for NATO members to increase defense spending to agreed upon levels, saying "each nation" has a duty to make the case to their own people about "why these resources are important to keep not only our own countries, but our alliance strong."