Secretary of State John Kerry is expected to take Turkey's prime minister to task for describing Zionism as a crime against humanity, an attack on a key U.S. ally that has overshadowed their talks on the crisis in neighboring Syria.
Kerry, on his first trip to a Muslim nation since taking office, is meeting Turkish leaders for talks meant to focus on Syria's civil war and bilateral interests from energy security to counter-terrorism.
But comment by Turkish Prime Minister Tayyip Erdogan at a U.N. meeting in Vienna this week, condemned by his Israeli counterpart, the White House and U.N. Secretary-General Ban Ki-moon, has clouded his trip.
"This was particularly offensive, frankly, to call Zionism a crime against humanity ... It does have a corrosive effect (on relations)," a senior U.S. official told reporters as Kerry flew to Ankara.
"I am sure the secretary will be very clear about how dismayed we were to hear it," the official, who spoke on condition of anonymity, said.
"To state the obvious, it complicates our ability to do all of the things that we want to do together when we have such a profound disagreement about such an important thing."
Washington needs all the allies it can get as it navigates the political currents of the Middle East, and sees Turkey as the key player in supporting Syria's opposition and planning for the era after President Bashar al-Assad.
But the collapse of Ankara's ties with Israel have undermined U.S. hopes that Turkey could play a role as a broker in the broader region.
"The Turkey-Israel relationship is frozen," the U.S. official said. "We want to see a normalization ... not just for the sake of the two countries but for the sake of the region and, frankly, for the symbolism," he said.
"Not that long ago (you) had these two countries demonstrating that a majority Muslim country could have very positive and strong relations with the Jewish state and that was a sign for the region (of what was) possible."
Erdogan told the U.N. Alliance of Civilizations meeting in Vienna on Wednesday: "Just as with Zionism, anti-Semitism and fascism, it has become necessary to view Islamophobia as a crime against humanity."
Erdogan's caustic rhetoric on Israel has won applause from conservative supporters at home but raised increasing concern among Western allies.
"Erdogan's comments about Israel have become progressively more worrying," said Moshe Kantor, President of the European Jewish Congress. "This type of rhetoric is pushing Turkey further away from the West and Europe and closer to Iran."
Ties between Israel and mostly Muslim Turkey have been frosty since 2010, when Israeli marines killed nine Turks in fighting aboard a Palestinian aid ship that tried to breach Israel's blockade of the Gaza Strip.
In recent weeks, there has been a run of reports in the Turkish and Israeli media about efforts to repair relations, including a senior diplomatic meeting last month in Rome and military equipment transfers.
The reports have not been confirmed by either government.
SUPPORT FOR SYRIAN OPPOSITION
Turkey's relations with the United States have always been prickly, driven more by a mutual need for intelligence than any deep cultural affinity. And Erdogan's populist rhetoric, sometimes at apparent odds with U.S. interests, is aimed partly at a domestic audience wary of Washington's influence.
But the two have strong common interests. Officials said Syria would top the agenda in Kerry's meetings with Erdogan and President Abdullah Gul, building on the discussions in Rome between 11 mostly European and Arab nations within the "Friends of Syria" group.
After the Rome meeting, Kerry said on Thursday the United States would for the first time give non-lethal aid to the rebels and more than double support to the civilian opposition, although Western powers stopped short of pledging arms.
"We need to continue the discussion which took place in Rome ... in terms of the main goals there is no daylight between us and the Americans," a senior Turkish official said.
"A broad agreement was reached on supporting the opposition. Now our sides need to sit down and really flesh out what we can do to support them in order to change the balance on the ground," he said.
Turkey has been one of Assad's fiercest critics, hosting a NATO Patriot missile defense system, including two U.S. batteries, to protect against a spillover of violence and leading calls for international intervention.
It has spent more than $600 million sheltering refugees from the conflict that began almost two years ago, housing some 180,000 in camps near the border and tens of thousands more who are staying with relatives or in private accommodation.
Washington has given $385 million in humanitarian aid for Syria but U.S. President Barack Obama has so far refused to give arms, arguing it is difficult to prevent them from falling into the hands of militants who could use them on Western targets.
Turkey, too, has been reluctant to provide weapons, fearing direct intervention could cause the conflict to spill across its borders.