Turkey on Wednesday said it would take military action against a US-backed "terrorist army" in Syria.

The Pentagon, meanwhile, sought to downplay training of Kurdish forces there, something that has enraged Ankara.

President Recep Tayyip Erdogan has warned in recent days of an imminent offensive on the Kurdish-held enclave of Afrin in northwestern Syria.

Military convoys have moved toward the border with Afrin this week.

Turkish long-range artillery has also been firing on Kurdish positions in Afrin.

Turkey's National Security Council said in a statement after a four-hour meeting that Turkey would act "swiftly and with determination" to eliminate threats posed by a "terrorist army."

Afrin is controlled by the Peoples' Protection Units (YPG), a Kurdish militia that Ankara considers a terrorist organization linked to the Kurdistan Workers' Party (PKK) waging a four-decade insurgency inside Turkey.  The US and EU consider the PKK a terrorist organization.

The YPG, however, is the main component of the US-backed Syrian Democratic Forces (SDF), which also include Arabs. The SDF has been at the forefront of the fight against the "Islamic State" (IS).

The United States over the weekend announced plans to create a 30,000-strong internal security and border security force, half of which would be SDF fighters.

The plan has angered Turkey, which has long threatened military action against the US-backed SDF in Afrin and other areas in northern Syria. 

Russia and Syria also condemned the US plan as a violation of Syria's sovereignty.

Read more: Turkey-US relations 'entering a risky period' 

Pentagon responds

In a statement on Wednesday, the Pentagon sought to assuage Ankara's concerns

"The US continues to train local security forces in Syria. The training is designed to enhance security for displaced persons returning to their devastated communities. It is also essential so that IS cannot reemerge in liberated and ungoverned areas," the Pentagon said.

It said a new "army" or conventional "border guard" force were not being created.

"We are keenly aware of the security concerns of Turkey, our Coalition partner and NATO ally. Turkey's security concerns are legitimate. We will continue to be completely transparent with Turkey about our efforts in Syria to defeat IS and stand by our NATO ally in its counter-terrorism efforts," it said.

Turkish fears

The SDF controls about 25 percent of Syria's territory in the north and northeast after pushing back IS on multiple fronts with the help of US Special Forces, arms and air power.

The rapid successes of the SDF have raised concerns in Turkey about what an endgame in Syria will look like now that the fight against IS has de-escalated and the Assad regime has also retaken large parts of the country from rebels after Russia intervened militarily in late 2015. 

The Syrian Kurds have created facts on the ground and set up autonomous structures for what they envision to be a future federal structure in Syria. They claim they have no desire for independence. 

But Turkey fears the empowerment of Syrian Kurds will have a knock-on effect on its own Kurdish minority. Ankara also wants to prevent what it views as an embryonic PKK statelet on its southern border. 

From Ankara's viewpoint, Afrin is a point of leverage over the Syrian Kurds because it is noncontiguous to areas under their control in the northeast. 

Tillerson signals US to stay in Syria

On Wednesday, Secretary of State Rex Tillerson signaled that the US forces would remain in Syria for the foreseeable future, in effect giving the Kurdish-led SDF and its political wings continued military and political support.

"Let us be clear, the United States will maintain a military presence in Syria focused on ensuring IS cannot re-emerge," said Tillerson in remarks delivered at Stanford University.

Outlining broader American goals, Tillerson said Washington also sought to reach a political solution in Syria, counter Iranian influence in the country and ultimately get Syrian President Bashar al-Assad to step down.

In another sign that the US will continue to back SDF controlled areas, Tillerson also said that Washington would not support reconstruction in Syria in areas under Assad's control.

"Instead, we will encourage international assistance to rebuild areas the global coalition and its local partners have liberated," he said, referring to the SDF.

Turkey preps offensive

Threats of an imminent military intervention against Afrin comes after Turkish-backed rebels and the Turkish military in October moved into parts of the northwestern Syrian province of Idlib to implement a "de-escalation" zone.

The move was part of a deal with Russia and Iran, both key backers of the Assad regime. Idlib is one of the last major rebel pockets in Syria and is largely controlled by an al-Qaeda-affiliated rebel alliance.

The Turkish military has set up observation posts and other positions in Idlib just south of YPG positions in Afrin, which is effectively surrounded by Turkey and Turkish-backed rebels.  

Rebels backed by the Turkish military carved out a "safe zone" in northern Aleppo province earlier last year after a months-long offensive that pushed IS from a section of the border.

The Turkish-backed military operation, known as Operation Euphrates Shield, was designed to remove IS from a section of the border and block the SDF from connecting areas under their control in the east with Afrin further to the west.