NATO's chief landed in Turkey on Friday amid a growing rift between the alliance's second largest military and its allies on both sides of the Atlantic.

Turkey faced widespread condemnation this week when it invaded Syria, a move that came after President Donald Trump withdrew troops from the region.

Trump has been accused of betraying the Kurds, who fought alongside the United States against ISIS and now face being cleared out by Turkish forces.

Turkey's incursion has already caused civilian deaths and risked what the United Nations warned could be a fresh humanitarian crisis in the region. Some fear it could also see the release of thousands of ISIS prisoners being guarded until now by Kurdish forces.

"While Turkey has legitimate security concerns, I expect Turkey to act with restraint," Jens Stoltenberg, NATO's secretary general, said in a speech in Istanbul on Friday. "Turkey is a great power in this great region. And with great power comes great responsibility."

Jens Stoltenberg, left, shakes hands with Turkey's President Recep Tayyip Erdogan in Istanbul on Friday. Presidential Press Service via AP

Tension between Turkey and the West is not new but this week perhaps marked a new low.

"Turkey has always been one of the most awkward members of NATO. The problem now is that the confrontation is at a much higher level," said Jonathan Eyal, international director of the Royal United Services Institute, a think tank based in London.

The problem for NATO is that while it may feel it needs to be seen to criticize Turkey's actions, the country is incredibly important for the military alliance.

Turkey boasts NATO's second largest army behind the United States, and sits on a crucial geographical crossroads between Europe, the Middle East, Russia and Central Asia.

Its airbases host U.S. nuclear weapons, and it proved a crucial player in the 2015 European migration crisis, with Turkey now holding some 3.6 million refugees.

"No other ally has suffered more terrorist attacks. No other ally is more exposed to the instability, violence and turmoil from the Middle East. And no other NATO ally hosts so many refugees as Turkey does, many of them from Syria," the secretary general said.

This tension was apparent in the response of Turkey's defense minister, Mevlüt Çavuşoğlu. "It is not enough to say 'we understand Turkey's legitimate concerns,' we want to see the solidarity very clearly," he said Friday.

Turkey has tried to portray the invasion as a counterterrorism operation, saying it is only targeting Kurdish groups that it considers terrorist organizations, as well as ISIS.

But the backlash from allies has been prompt and severe.

Turkish armored vehicles drive along their side of the Syrian border on Friday. Nazeer Al-Khatib / AFP - Getty Images

Twenty nine House Republicans plan on introducing legislation to sanction Turkey, and the European Union will discuss a similar move at a leaders' meeting next week.

Norway announced Thursday it had suspended arms sales to Turkey because of "the rapidly changing situation on the ground," its foreign ministry said in an emailed statement.

Amelie de Montchalin, France's minister for European Union affairs, even suggested that suspending Turkey's NATO membership was an option. She made the comments to France Inter radio, according to a translation by Bloomberg.

Acrimonious as this may seem, some experts believe that there is little risk to Turkey's role in the alliance itself.

The U.S. and Turkey have rubbed up against each other over various issues in recent years, including Washington's relationship with Kurdish groups in Syria that Ankara considers to be terrorists.

Things became especially fraught in July when Washington kicked Turkey off the F-35 fighter jet program. This was because Ankara chose to buy an advanced Russian missile defense system over its U.S. equivalent.

Although members might not like it, there is nothing in NATO's charter that says they have to agree with their allies' actions, said Professor Peter Roberts, who served as an officer with the British Royal Navy and served on several NATO working groups.

"NATO occupies such a critical geostrategic and geopolitical location, it's almost impossible to foresee an enormous change in the way that the alliance, as a military political alliance, will interact with Turkey," said Roberts said, who is now director of military sciences at the Royal United Services Institute.