Ankara, Turkey - US National Security Adviser John Bolton has arrived in Turkey to try to hammer out a deal for Kurdish fighters in Syria, as the United States prepares a military withdrawal from the country.

Bolton touched down in the Turkish capital, Ankara, on Monday, and will be joined by US Joint Chiefs of Staff General Joseph Dunford, having stated the US pullout is conditional on defeating the Islamic State of Iraq and the Levant (ISIL) group, as well as Turkish assurances on the security of the US's Kurdish militia allies.

President Donald Trump's December 19 announcement of the withdrawal of 2,000 troops from Syria has heightened expectations that Turkey could launch a military operation targeting the People's Protection Units (YPG), which has spearheaded the US-allied Syrian Democratic Forces (SDF) in the fight against ISIL.

"We don't think the Turks ought to undertake military action that's not fully coordinated with and agreed to by the United States," Bolton told reporters in Jerusalem on Sunday.

He added Turkey must "meet the president's requirement that the Syrian opposition forces that have fought with us are not endangered."

The US alliance with the YPG, which it has helped arm and train, has been one of the main stumbling blocks in relations with Ankara because of the YPG's ties to the Kurdistan Workers' Party (PKK). Since 1984 the PKK has waged a war against Turkey that has led to more than 40,000 deaths.

Ankara regards both the Kurdish groups as "terrorist" organisations.

Turkey-YPG tensions

Responding to Bolton's remarks, Turkish Presidential Spokesman Ibrahim Kalin maintained Turkey's opposition to the YPG, which he described as oppressing Syrian Kurds.

"That a terror organisation cannot be allied with the US is self-evident," he said in a statement carried by Turkey's Anadolu news agency.

Ziya Meral, director of the London and Beirut-based Centre on Religion and Global Affairs, said it would be "naive" for the US to expect Turkey not to "interfere" in Syria while the YPG remained in control across the border.

"At best, the Trump administration can achieve an orderly pullout process and prevent a short-term military intervention against YPG," Meral said. "Turkey would not want to be a single actor advancing deep into Syrian territory to defeat Islamic State, but would want US and coalition support."

Turkey has launched two military operations in northern Syria since 2015 that have targeted the YPG. Even before the US withdrawal announcement, President Recep Tayyip Erdogan pledged to attack YPG-controlled territory east of the Euphrates River.

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Last month the US set up observation posts along the Turkish border after Turkey shelled YPG positions.

Washington claimed the deployment was to safeguard Turkey's security while Erdogan said it was to protect the YPG.

Over the course of the war, the group has gained control of the northeastern quarter of Syria that borders Turkey and Iraq.

In recent weeks, Turkish troops and tanks have been deployed to the border and have reinforced Turkey-backed militias around Manbij, a town to the west of the Euphrates that the YPG captured from ISIL in 2016.

The YPG has turned to the Syrian regime as a counter to the threat from Turkey in the event of a US retreat. The YPG withdrew from Manbij last Wednesday as part of a deal with President Bashar al-Assad's government, the official Syrian Arab News Agency reported.

American assurances

Bolton said Jim Jeffrey, the US special representative for Syria and the anti-ISIL coalition, will travel to Syria this week to reassure Kurdish fighters of continued US support.

He called on the YPG to refrain from seeking Russian or Syrian government protection. "I think they know who their friends are," Bolton said, referring to the Kurds.

The US visit comes amid anger in Turkey over comments by Secretary of State Mike Pompeo, in which he said the US wanted to prevent Turkey "slaughtering" Kurds in Syria.

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His remarks to online outlet Newsmax led to a sharp rebuke from the Turkish Foreign Ministry, which issued a statement on Friday accusing Pompeo of showing "a worrying lack of information" in equating the YPG to all Syrian Kurds.

Bolton said his meetings with Turkish officials, including Erdogan, would seek to clarify uncertainty around Turkey's "objectives and capabilities" in Syria.

Turkey has pledged to continue the fight against ISIL in Syria after the US withdraws, although Ankara has reportedly requested significant US military support to carry out the mission.

"The US finds itself in an unsustainable position, which was set to happen the moment it chose to arm the PKK's Syria offshoot," said Meral.

"For Ankara, talks with Russia, thus indirectly with the Assad regime, are as important as talks with the US. Ultimately Ankara too needs an exit strategy as it wants Syria to remain intact."