The Pentagon’s top leader says Turkey’s plan to purchase Russian surface-to-air-missile systems raises questions about NATO compatibility and deployment.

Turkey, a NATO ally, last week agreed to purchase four of the Russian-made S-400 mobile missile batteries for $2.5 billion over the next few years.

“The problem is, is how do you interoperate with NATO systems with Russians — they’ll never interoperate,” Defense Secretary Jim Mattis said Friday after stopping in the Pentagon press room to speak with reporters before a meeting with Secretary of State Rex Tillerson to discuss options in Syria.

The advanced system, known as the “Triumf,” has been spotted in Syria and can carry multiple, short- to very long-range missiles with a variety of sensor systems. It has a range of 400 kilometers and is effective against stealth aircraft, ballistic missiles, and cruise missiles, according to the Congressional Research Service.

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“So we’ll have to see, does it go through? Do they actually employ it? Do they only employ it in one area? But we’ll have to take a look at it; Obviously, it’s not going to be interoperable,” Mattis said.

How Turkey intends to use the system hasn’t been spelled out, but the planned purchase comes at a time when Turkey and the U.S., and other European countries, find themselves in a complex military conflict in Syria.

The U.S. backs the Kurdish militia fighting in Syria, and in May began sending small arms to the group. Meanwhile, Turkey considers the Kurdish YPG, or the People’s Protection Units, a terrorist group linked to the PKK, or Kurdistan Workers Party, which has carried out attacks in southeastern Turkey. The U.S. considers the YPG the most effective anti-ISIS force in northeastern Syria.

The channels between Russia and Turkey, however, have been slowly expanding.

Last year, Turkey’s President Tayyip Erdogan traveled to Russia to speak with President Vladimir Putin about Russia’s interests in Syria. Tensions between the two countries eased after Erdogan formally apologized to Putin for the shoot down of a Russian Sukhoi Su-24 bomber aircraft over Turkish airspace in 2015. Before their meeting in August, Putin also offered support to Erdogan after a faction within the Turkish armed forces attempted to overthrow the government — but failed.

Mattis reiterated the U.S. had no formal say in how Turkey chose, or will proceed with the SAM system.

“This is a sovereign decision,” he said.

Another Ceasefire Zone in Syria

Mattis also said Friday there may be another Syria ceasefire zone in coming weeks.

“I do know where it is, [but] I’d rather not go into it right now, to leave the diplomats the maneuver room that they need,” he said.

The ceasefire imposed July 9 “appears to be holding,” Mattis said, noting it’s the first time one has held this long. The recent ceasefire was brokered by the U.S., Russia and Jordan and imposed days after President Donald Trump met with Putin in Germany during the G20 Summit.

Past ceasefire negotiations have only lasted a few short days, the defense secretary said, but this time “there’s reason for optimism.”

“I think once you get one in you have to look at where…it can be expanded,” he said.

If the U.S. military will have a role in the next ceasefire zone remains to be seen. It depends “on what groups are in the area,” Mattis said.

Last month, a U.S. Navy F/A-18E Super Hornet shot down a Syrian Su-22 after the Soviet-era fighter-bomber dropped munitions near U.S.-backed Syrian Democratic Forces fighters. The attack came after pro-Syrian forces attacked SDF fighters in Ja’Din, a town south of Tabqah and a known area where U.S. works with Russia to deconflict the airspace.

Similarly, a U.S. F-15E on June 8 shot down an unidentified drone deemed hostile toward coalition forces in At Tanf. The strike marked the first time that forces supporting the Syrian government had attacked inside a so-called “deconfliction” zone near At Tanf, close to the Jordanian border.

Mattis has previously said the hostile, pro-Syrian forces are backed by Iran, and have been knowingly operating “inside an established and agreed-upon deconfliction zone.” They are believed to be a threat to coalition forces in the region, he has said.

When asked if a ceasefire zone in the south, near At Tanf seemed like a logical choice, Mattis deflected.

“You look for opportunities right now,” he said. Generally speaking about the Syrian war, Mattis said, “This has got to stop.”