James Jeffrey, US Special Representative for Syria Engagement, Turkey's Defense Minister, Hulusi Akar, repeated his country's opposition to the observation posts, even demanding that the US end its relationship with the YPG.
"We will start the operation in east of the Euphrates in a few days to save it from the separatist terrorist organization", Erdoğan added, referring to the YPG. "Our target is never USA soldiers", Erdogan said at a speech at a defense industry summit in Ankara.
"This step will allow for the path to a political solution to be opened and for healthier cooperation", Erdogan added.
Pentagon spokesman Commander Sean Robertson said any unilateral military action in northeast Syria would be a "grave concern", as it could potentially jeopardise USA troops working with the SDF in the region.
In its more than 30-year terror campaign against Turkey, the PKK - listed as a terrorist organization by Turkey, the US and the European Union - has been responsible for the deaths of some 40,000 people, including women and children.
On Tuesday and Wednesday, Turkey sent convoy of troops and military equipment to the border with Syria as reinforcements, according to the state-run Anadolu Agency. The Turkish currency has been among the world's worst performers this year, partly due to escalating tensions with the USA on a range of issues, from Syria policy and Erdogan's growing ties with Russian Federation to Turkey's detention of an American pastor.
"We know that the aim of the radar and observation posts set up by the United States is not to protect our country from terrorists, but to protect the terrorists from Turkey".
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A possible mission east of the Euphrates, which Turkey's leadership has been suggesting for months, would follow two successful cross-border Turkish operations into Syria since 2016 - Operation Euphrates Shield and Operation Olive Branch - both meant to eradicate the presence of YPG/PKK and Daesh terrorists near Turkey's borders.
"This is more a messaging effort to push back against the US claim that these border posts were for Turkey's protection, rather than, as is clearly the case, to protect the SDF from Turkey", he told VOA. The official said the positions were clearly marked and any force attacking them "would definitely know they are attacking the United States".
Other analysts, like Kyle Orton, who is a Syria researcher in London, believe that while Turkey is capable of carrying out an attack in northeastern Syria, Erdogan's rhetoric has generally been cautious regarding Syria, especially in areas where there is a US presence.
Erdogan had claimed that ISIS is no longer a threat in northern Syria during his speech on Wednesday.
Erdogan said on Wednesday that the military offensive that has been so far conducted on the western side of the Euphrates River will now target the east. "The fact that we have deep differences in perception with the United States is no secret", he said.
The U.S. makes a distinction between the PKK and YPG, however, providing military support to the latter in its fight against IS militants in Syria.
Meanwhile, the Britain-based Syrian Observatory for Human Rights, a war-monitoring group, said at least four people were killed in two explosions in Syrian towns held by Turkey-backed local administrations shortly after Erdogan's announcement.
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