Washington - The United States is on a collision course with its Nato ally Turkey, pushing ahead with arming Syrian Kurds after deciding the immediate objective of defeating Islamic State (ISIS) militants outweighs the potential damage to a partnership vital to US interests in the volatile Middle East.The Turks are fiercely opposed to the US plans, seeing the Kurdish fighters as terrorists. And when Turkish President Recep Tayyip Erdogan visits the White House on Tuesday, the most he and President Donald Trump may be able to do is agree to disagree, and move on."The Turks see this as a crisis in the relationship," said Jonathan Schanzer at the Washington-based Foundation for Defence of Democracies.The challenge is hardly new. Long before Trump took office, US presidents have grappled with the fragility of partnering with Turkey's government and the Kurds to carry out a Middle East agenda.Past administrations have sought a delicate balance. Too exuberant in its support for the Kurds, and the US risks pushing ally Turkey toward US geopolitical rivals like Russia or emboldening the Kurds to try to create an independent state - a scenario that would destabilise multiple countries in the region. Too little co-operation with the Kurds risks squandering a battlefield ally with proven effectiveness against extremist threats and who has staunchly supported Washington.Trump has made his priorities clear.His administration is arming Syrian Kurdish fighters as part of an effort to recapture the Syrian city of Raqqa, the Islamic State group's self-declared capital. Coupled with the US-backed fight in the Iraqi city of Mosul, Raqqa is seen as a key step toward liberating the remaining territory the militants hold.Turkey has been pressuring the US to drop support for the Kurdish militants in Syria for years and doesn't want them spearheading the Raqqa effort. Turkey considers the Syrian Kurdish group, known as the YPG, a terrorist group because of its ties to the outlawed Kurdish Workers' Party inside Turkey. The United States, the European Union and Turkey all agree the PKK is a terrorist organisation.The Turks fear any weapons the US provides the Syrian Kurds could well end up with their ethnic brethren in Turkey, who've fought violently as part of a separatist insurgency for more than three decades. As a nod to Turkey's concerns, the Pentagon has promised tight monitoring of all weapons and greater intelligence sharing to help the Turks better watch over their frontiers. Kurds are an ethnic group predominantly concentrated along the borders of four countries - Turkey, Syria, Iraq and Iran.Wish list But a face-to-face confrontation on the matter between Trump and Erdogan seems inevitable.Erdogan and other top Turkish officials have pressed for the US to reverse its strategy, however low the prospects of Trump changing his mind. As a result, experts see Erdogan using the meeting to confront Trump on a host of other Turkish grievances. Those include extraditing the Pennsylvania-based cleric, Fethullah Gulen, whom Erdogan blames for fomenting a failed coup last summer, and dropping US charges against Reza Zarrab, a Turkish businessman accused of money-laundering and violating US sanctions in Iran."I see this trip as a new milestone in Turkey-US relations," Erdogan said, as he prepared to fly to Washington.The US, too, has a wish list for Turkey. Washington is concerned by rising anti-Americanism in Turkey that Erdogan's government has tolerated since the July coup attempt. The US also has pressed unsuccessfully for the release of Andrew Brunson, an American pastor, and other detained US citizens.Trump also has much at stake. His willingness to partner with authoritarian rulers and overlook their shortcomings on democracy and human rights have alarmed US lawmakers of both parties. Trump's premise has been that he is focusing on deal-making. That puts added pressure on him to get results.Trump has gone out of his way to foster a good relationship with Erdogan. After a national referendum last month that strengthened Erdogan's presidential powers, European leaders and rights advocates criticised Turkey for moving closer toward autocratic rule. Trump congratulated Erdogan.Now, the American leader may try to cash in."Trump has prioritised protecting US national security interests over lecturing allies on democratic values or human rights," said James Phillips, a senior research fellow for Middle Eastern affairs at the Heritage Foundation. "I don't think the president will lose any sleep if he is criticised for meeting with President Erdogan, as long as it pays dividends for advancing his foreign policy agenda."But Erdogan may not be amenable to accepting the US military support for the Kurds in a quid pro quo. Last month, the Turkish military bombed Kurdish forces in Syria and Iraq, in one case with American forces only about 10km away. His government has insisted it may attack Syrian Kurdish fighters again. The US, whose forces are sometimes embedded with the Kurds, has much to fear.Barack Aydin of the Washington-based Kurdish Policy Research Centre, said the key ought to be a broader peace process between Erdogan's government and Kurdish opponents in Turkey, which would eliminate these problems."That would be a very good start," Aydin said.