US Secretary of State John Kerry was due to discuss the Syria crisis with Turkey's leaders on Thursday in talks likely to be strained by controversial comments by Prime Minister Recep Tayyip Erdogan branding Zionism a "crime against humanity."
The talks come a day after Washington announced that it would for the first time provide direct aid to Syrian rebels in the form of food and medical supplies as well as $60 million in extra assistance to the political opposition.
The two-year civil war in neighbouring Syria which has killed at least 70,000 people according to the UN will top the agenda for Kerry, who is due to meet with President Abdullah Gul, Erdogan and Foreign Minister Ahmet Davutoglu.
But the talks are likely to be overshadowed by renewed tension between Turkey and Israel, two major Washington allies, following comments Erdogan made earlier this week at a UN-sponsored forum in Vienna.
"As is the case for Zionism, anti-Semitism and fascism, it is inevitable that Islamophobia be considered a crime against humanity," Erdogan said.
His comments were branded as "a dark and mendacious statement," by Israeli Prime Minister Benjamin Netanyahu while Washington said "characterisation of Zionism as a crime against humanity... is offensive and wrong."
A US official travelling with Kerry on his first trip abroad since taking over as secretary of state from Hillary Clinton said he would raise the issue with his hosts.
"We put out a statement from Washington making clear that the statement was both offensive and wrong and I am sure the secretary will be able to convey that to the prime minister directly this afternoon," said the official who requested anonymity.
"We have been working hard over the last couple of years to foster a better cooperation to restore what was once an historic cooperation between Turkey and Israel," he added.
Turkish-Israeli relations have remained in free-fall since Israeli troops raided a Gaza-bound Turkish aid ship in 2010, killing nine people.
The incident strained bi-lateral military and diplomatic ties and left Washington in a bind to mend relations between its two fundamental allies in the region.
Turkey, once a close ally of Syria, has joined the US in its campaign to oust the regime of President Bashar al-Assad and has given shelter to Syrian rebels and to nearly 200,000 refugees along its volatile border.
In January, the United States began deploying Patriot missiles, along with Germany and the Netherlands, as part of a NATO mission to protect Turkey from any spillover of the Syrian conflict.
Turkey had requested the measure after several cross-border shelling incidents, including one in October that killed five civilians.
Kerry was also due to attend a memorial ceremony in Ankara, in honour of the US embassy guard who was killed in a February 1 suicide attack claimed by the fiercely anti-US Marxist group the Revolutionary People's Liberation Front (DHKP-C.)
Among the issues on his agenda during the talks is also Washington's pressure for increasing sanctions on Iran due to its disputed nuclear programme.
Iran is Turkey's second-biggest natural gas supplier after Russia, and third biggest in oil. Turkey says it is bound by only UN sanctions against Tehran imposed by the United States and the European Union.
The situation of jailed journalists in Turkey -- which has more imprisoned reporters than any other country in the world -- is also expected to be raised by Kerry.
Some 76 journalists were jailed in Turkey as of August 1, including 61 who were put behind bars purely because of their journalism, according to a report by the Committee to Protect Journalists.