US Secretary of State Rex Tillerson visited Turkey twice during his first year in office, but his third trip due next week may well be the most important.

Turkish-US relations were strained during former President Barack Obama's tenure. Some observers say they've only become worse during President Donald Trump's administration - and they fear the NATO allies could clash militarily in Syria.

Tillerson's mission is to reduce the tensions.

Like Obama before him, Trump is backing the Kurdish-led Syrian Democratic Forces (SDF) because of their success in reclaiming territory once held by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.

That has angered Turkish President Recep Tayyip Erdogan.

The SDF is an umbrella group of fighters led by the People's Protection Units (YPG) - a Kurdish rebel group considered a "terrorist group" by Ankara.

Erdogan routinely accuses Washington of undermining his country's security by supporting Kurdish fighters inside Syria, refusing to extradite cleric Fethullah Gulen for allegedly plotting a failed coup attempt in Turkey in 2016 and for never trying to overthrow Syrian President Bashar al-Assad.

More worrisome for the US, Erdogan launched "Operation Olive Branch" in late January, sending his forces into the northwestern Syrian town of Afrin in order to prevent YPG rebels from taking control.

He is now threatening to send his troops east to Manbij, where the SDF and US troops are securing the city from ISIL fighters.

The US doesn't want that to happen, and Tillerson will repeat that message during his meetings.

On Friday, a senior state department official told reporters: "We are urging them to show restraint in their operations in Afrin, and to show restraint further along the line across the border in northern Syria."

"We need to work with the Turks more to figure out a way if there's a way that we can work with them to address their legitimate security concerns while, at the same time, minimising civilian casualties and above all else keeping everything focused on the fight against ISIL, which is not over," the official said.

But analysts say, on this trip, it's more important for Tillerson to listen than to talk.

Senior policy analyst Nicholas Danforth, of the Bipartisan Policy Center, told Al Jazeera he was cautiously optimistic.

"I continue to think that on both sides, sanity will prevail; that both sides will realise the damage they will suffer from a breakdown of the US-Turkish relationship, the NATO alliance, would be catastrophic, and therefore they will avoid provocative steps," he said.

"But again, rhetorically, on the ground, you have both sides working themselves into positions that will be increasingly difficult to back down from."

That fact will make Tillerson's third visit to Turkey one to watch.