The U.S. defense secretary says Russian leaders need to contact the Pentagon right away to talk about Moscow's military activities in Syria.

Defense Secretary Ash Carter says the U.S. is waiting for a second meeting with Russia on how to avoid any accidents or miscalculations in the increasingly crowded skies over Syria.

U.S. and Russian officials talked by video conference last week. Then at least one Russian warplane violated Turkey's airspace over the weekend. NATO has denounced Russia for "irresponsible behavior."

Speaking at an air base in Spain, Carter said Tuesday that Russia's failure to respond to the U.S. meeting request may be "a further sign of their strategic confusion."

A U.S.-led coalition has been conducting airstrikes on Islamic State militants in Syria. Russia began airstrikes there last week.