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"This is unacceptable." That's what NATO Secretary-General Jens Stoltenberg had to say about Russian military aircraft violating Turkey's airspace twice this weekend. Stoltenberg also has said he doubts Russia's explanation that it was an accident.

At issue most recently is the Turkish military's allegation that on Sunday, "a MiG-29 plane of unidentified nationality for five minutes and 40 seconds kept two Turkish F-16 planes on its radar as potential targets," reports the Russian news agency Tass.

Turkey's foreign minister says his country has now twice summoned Russia's ambassador to protest the incursions — one on Saturday and another on Sunday.

Stoltenberg said he has convened a special meeting over the airspace violations, and that NATO is in "strong solidarity with Turkey."

"I'm also concerned that Russia is not targeting ISIL," Stoltenberg said, "but instead attacking the Syrian opposition and civilians."

When Russia began its airstrikes and patrols to support Syrian President Bashar al-Assad last week, the country was quickly criticized for targeting positions held by U.S.-supported rebels rather than those of the so-called Islamic State.

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