Turkey's President Recep Tayyip Erdogan speaks during a news conference in Riga October 23, 2014. Divergences over how to confront the Islamic State have strained the durability of a six-decade alliance. (Ints Kalnins/Reuters)

The increasingly hostile divergence of views between Turkey and the United States over Syria is testing the durability of their 60-year alliance, to the point where some are starting to question whether the two countries still can be considered allies at all.

Turkey’s refusal to allow the United States to use its bases to launch attacks against the Islamic State, quarrels over how to manage the battle raging in the Syrian border town of Kobane and the harsh tone of the anti-American rhetoric used by top Turkish officials to denounce U.S. policy have served to illuminate the vast gulf that divides the two nations as they scramble to address the menace posed by the extremists.

Whether the Islamic State even is the chief threat confronting the region is disputed, with Washington and Ankara publicly airing their differences through a fog of sniping, insults and recrimination over who is to blame for the mess the Middle East has become.

At stake is a six-decade-old relationship forged during the Cold War and now endowed with a different but equally vital strategic dimension. Turkey is positioned on the front line of the war against the Islamic State, controlling a 780-mile border with Iraq and Syria. Without Turkey’s cooperation, no U.S. policy to bring stability to the region can succeed, analysts and officials on both sides say.

“If Turkey is not an ally, then we and Turkey are in trouble,” said Francis Ricciardone, who served as the U.S. ambassador to Turkey until the summer. “It is probably the most important ally.”

The airdrop by U.S. warplanes last week of weapons to a Kurdish group Turkey regards as a terrorist organization crystallized the apparent parting of ways. Turkish President Recep Tayyip Erdogan has not disguised his anger at the way President Obama ordered the airdrop. The U.S. president informed him of the decision in a telephone call barely an hour after Erdogan had declared to journalists that Turkey would never allow such assistance to take place.

On a tour of the Baltic states last week, Erdogan blasted Obama at every stop. “Mr. Obama ordering three C-130s to airdrop weapons and supplies to Kobane right after our conversation cannot be approved of,” he said during a news conference in Latvia. “The U.S. did that despite Turkey,” he fumed on another leg of the journey.

U.S. officials have sought to reassure Turkey that the airdrop was a one-time action, and the two countries have agreed on a plan to reinforce the beleaguered Syrian Kurds with Iraqi peshmerga fighters, which Turkey does not object to, because it has friendly relations with Iraqi Kurds.

But the Kobane dispute masked more fundamental differences over a range of issues, some of which have been brewing for years and others that have been brought to light by the urgency of the U.S.-led air campaign, analysts say.

“The Syria crisis is exposing long-unspoken, unpleasant truths about the relationship that were put to one side,” said Bulent Aliriza, a Turkish analyst with the Washington-based Center for Strategic and International Studies. “We have this kabuki dance where Washington and Ankara say they agree, but they don’t.”

The tensions are not unprecedented, nor are the doubts about an alliance born in a different era, when fears of Soviet expansionism brought Muslim Turkey under NATO’S umbrella and extended the Western bloc’s reach into Asia.

The United State imposed an arms embargo on Turkey after Turkish troops invaded Cyprus in 1974. In 2003, there was fury in Washington when Turkey’s parliament refused to allow American troops to use Turkish soil as a staging ground for the invasion of Iraq, triggering a deep chill that took years to overcome.

The 2003 rupture may, however, have foreshadowed the beginning of a more fundamental shift in the relationship, with Erdogan embarking on a decade of transformation in Turkey that has perhaps forever changed his country, analysts say. Turkey has grown and prospered under his rule, but it has also begun to tilt toward a more authoritarian, Islamist brand of politics that is increasingly at odds with the model of secularism and pluralism that the United States has held up as a key component of Turkey’s importance to the alliance.

In 2003, as now, Turkey made it plain it did not want to be used as a launching pad for attacks against fellow Muslims in the Middle East, a sentiment Erdogan has repeatedly expressed in his many recent comments critical of U.S. policy. He has accused the United States of being more interested in oil than in helping the people of the region and has made it clear that he does not regard the Islamic State as a greater threat than the Kurdistan Workers’ Party, or PKK, the organization affiliated with the Kurdish Syrians the United States has been helping in Kobane.

“There are growing doubts over whether the U.S. and Turkey share the same priorities and even whether they share the same goals,” Aliriza said. “Even when it comes to defining the enemy — there is no common enemy.”

Turkish officials bristle at suggestions that Turkey is in any way sympathetic to the Islamic State. It is Turkey that has to live with the jihadist group on its borders, not the United States, and Turkey that is most at risk of being targeted by the Islamic State in retaliation for waging war against it, the officials say.

Turks also do not mask their irritation with what they regard as a shortsighted and potentially dangerous U.S. strategy that they believe will not work and could backfire. Turkey believes Syrian President Bashar al-Assad is the root cause of the instability that gave rise to the Islamic State and that leaving him in place will serve only to prolong the war, a senior Turkish official said, speaking on the condition of anonymity because he is not authorized to discuss policy on the record.

Turkey is hosting more than 1.5 million refugees, a huge social and financial burden that will continue to grow if the conflict in Syria is not resolved, the official said.

“They are across the Atlantic,” he said, referring to the United States. “We are a neighbor of Syria’s. We know that if Assad stays, the problem will continue for decades. The Americans have the luxury of cherry-picking the problems, but we need to see them as an entirety.”

Obama and other top U.S. officials have repeatedly said that Assad cannot be part of any long-term solution to the Syria problem. But, another Turkish official said, “saying it is one thing, and doing it is another.”

“Much, much more needs to be done,” the second official said, also speaking on the condition of anonymity. “To fix this region, we have to think big. We have to think long-term and have a holistic strategy underpinned by values that don’t change according to the season.”

U.S. officials acknowledge that Washington policymakers do not always sufficiently take into account the concerns of allies. They also point to areas where Turkey is expanding its cooperation, including restricting the flow of foreign fighters across its borders and identifying the networks in Turkey that support them.

“We’ve seen some steps recently where they are more engaged on both of those issues,” said a senior administration official in Washington, speaking on the condition of anonymity to discuss the sensitive diplomacy. “We’re definitely encouraged there.”

And in some ways, the Syria crisis has brought Turkey and the United States closer after a year of building tensions, officials on both sides say. Obama and Erdogan had not spoken since January until they met in Wales in September to discuss the formation of the anti-Islamic State coalition. Lower-level officials have since been talking multiple times a day, Turkish and U.S. officials say. Vice President Biden has announced plans to visit Turkey in November in an effort to smooth over the ruckus over comments he made suggesting that Turkey is responsible for the rise of the Islamic State.

It is hard, however, to avoid the impression that Turkey and the United States are moving on separate tracks — “parallel tracks that don’t converge,” said Gokhan Bacik, a dean at Ipek University in Ankara.

“From now on, this is only a relationship of necessity,” he said. “There is nothing ideologically that the United States and Turkey share. Turkey has changed.”

Karen DeYoung in Washington contributed to this report.