SDF leaders say that the militant group wants to control all the areas gained in the fight against Daesh, seeking autonomy.
A senior commander in the Syrian Democratic Forces says the SDF is in the process of rebuilding its forces.
The SDF's deputy commander Abdul Qader Effedili was quoted by the British newspaper, The Times, as saying: "We are rebuilding our own army to be ready to deal with any threat from anyone who wants to invade the cities we liberated alongside the coalition."
Omar Alloush, a leading member of the so-called civil council in Raqqa also told to The Times they want "some autonomy".
Since the Syrian conflict erupted in 2011, the YPG militants and its allies have carved out autonomous cantons in the north. The YPG spearheads the SDF, an alliance of Kurdish and Arab militants fought against Daesh with Washington's backing.
Effedili argued that Turkey, Iran, and Syrian regime forces would be happy to take back the SDF-controlled areas.
Effedili's comments follow reports earlier this week by the Anadolu Agency that a group of around 400 YPG militants were being reportedly trained by the US through the Pentagon and the CIA near eastern Aleppo's Tishrin Dam on the Euphrates River and in the southern Hasakah province.
The so-called "border guards" have formed what they call "The North Army," said the sources, who spoke to Anadolu Agency on condition of anonymity due to security concerns.
According to these sources, this new structure is to be composed mainly of the YPG, which Turkey accuses of having invaded Syrian territory on the pretext of fighting Daesh.
Turkey considers the PYD and the YPG as offshoots of the PKK, which has waged war against Turkey for more than 30 years. Turkey, US, and EU list the PKK as a terrorist organisation.
The US previously said they would continue to support the YPG in Syria's eastern Deir Ezzor province, home to some of the country's most productive oilfields.
The latest news comes after the US Central Command (CENTCOM) Commander General Joseph Votel announced on December 22, 2017 that they would establish border protection forces in Syria, which he said would help prevent the resurgence of Daesh.
Turkish Foreign Ministry on Wednesday summoned the US Embassy Charge d'Affaires Philip Kosnett in capital Ankara to express its "discomfort" over the US support for the YPG, according to a diplomatic source.
The US weapons training of YPG militants was discussed when Kosnett was summoned to the ministry, the source said, speaking on condition of anonymity due to restrictions on talking to the media.