Defense Secretary Jim Mattis has said support for the YPG might not end after the battle to drive Daesh out of Raqqa is accomplished, as the US had earlier promised.
Turkish Defence Minister Fikri Isik is meeting his American counterpart, James Mattis, in Brussels on Wednesday.
Their meeting comes ahead of a NATO Defence Ministers meeting on Thursday.
The US decision to arm the YPG, which is the armed wing of the PYD and the Syrian affiliate of the PKK, is expected to top the agenda. The PKK has been designated a terrorist organisation by Turkey, the EU and the US.
Turkey and the United States have been at odds for a long time over US military and financial support for the YPG.
During the administration of former President Barack Obama, Ankara offered Washington its cooperation in the US-backed campaign to drive Daesh out of its de facto Syrian capital, Raqqa.
His successor Donald Trump made no change to the Pentagon decision to use the YPG as the spearhead of the campaign against Daesh.
The move has angered Turkey. One of Ankara's concerns is that arms provided to the YPG could be transferred to the PKK, and US-trained YPG members could join PKK who have been fighting the Turkish state for more than 30 years.
Mattis sent a letter to Isik last week, saying the US would provide Turkey with a list of arms provided to the YPG to ensure the process was transparent.
The US defence secretary promised to prevent the YPG from passing on the weapons to the PKK and said the US would take back from the YPG any weapons supplied to them after the Raqqa offensive ended.
In his letter, Mattis stressed that US cooperation with the YPG was temporary and would end once Daesh was defeated in Raqqa.
However, en route to Germany ahead of the NATO meeting in Brussels, Mattis told reporters that Washington would continue to provide weapons to the YPG even after the battle for Raqqa ended.
When asked whether the US would take back from the YPG the weapons it supplied after the fight against Daesh ended, Mattis said it depended on when or where the next mission was.
In his meeting with Mattis on Wednesday, Isik will be aiming to clarify what America's real intention with the YPG was, and if they had promised more to the group in Syria.
Territorial integrity is key to Ankara's Syria policy. Turkey strongly opposes the idea of any autonomy or separate rule in YPG-controlled areas in Syria, which is on Turkey's southern border.
Fighting in Afrin
The US has military bases and advisers which are aiding the YPG in Syria, despite Turkey's strong opposition, and is based in YPG-controlled areas in Syria's northeast.
A second area the YPG controls is in the northwest, Afrin.
Between the two YPG-controlled areas is a region contolled by armed Syrian opposition groups backed by the Turkish army.
Last week, Turkey deployed additional trucks and soldiers to its border with Syria, near Afrin.
YPG attacked on Turkish-backed forces in the Afrin area on Tuesday, prompted retaliation from the Turkish military.
It was not the first time the Turkey army had fired on YPG positions in Syria. But tensions are rising in Afrin.
Turkish authorities, including President Recep Tayyip Erdogan, have been signalling for months about the possibility of a move against the YPG in Afrin.
The latest artillery bombardment on different locations in Afrin came after Mattis' remarks that the US could continue arming the YPG even after Daesh was driven out of Raqqa.
Afrin is surrounded by Turkey, Turkish-backed Syrian opposition-controlled areas and Syrian regime-held areas.
Complicating the situation further is Russia, which is providing financial support for the YPG in Afrin, in a bid not to lose its influence with the US-backed group.
As a result, Afrin has become a flashpoint in the tense northern border area in Syria.