By Daren Butler and Sarah Dadouch
ANKARA (Reuters) - Turkey may start a new military operation in Syria at any moment, President Tayyip Erdogan said on Monday, adding that the White House has responded positively to Ankara's plans to push into territory where its forces have not previously operated.
The Pentagon has expressed grave concern, saying unilateral military action by any party in northeast Syria - where U.S. forces operate - would be "unacceptable".
But Erdogan suggested that President Donald Trump was more receptive to Turkish plans to move east of the Euphrates river than his own U.S. Defense Department.
"We officially announced that we will start a military operation to the east of the Euphrates, "Erdogan said in a speech in the central province of Konya. "We discussed this with Mr Trump and he gave a positive response."
Turkey and the United States have long been at odds over Syria policy, where Washington has backed the Syrian Kurdish YPG militia against Islamic State. Ankara considers the YPG a terrorist organisation tied to the Kurdistan Workers Party (PKK), an outlawed group that has waged a three-decade insurgency in southeast Turkey.
Erdogan said last week that Ankara would launch a new campaign within days against the YPG, although he has yet to give a more definitive timeline.
Any new campaign is likely to be complicated by the presence of U.S. soldiers in northern Syria. Turkey has already intervened to sweep YPG and Islamic State fighters from territory west of the Euphrates over the past two years, but has not gone east of the river - partly to avoid direct confrontation with U.S. forces. The Pentagon says it has about 2,000 troops in Syria.
"We can start our operations on Syrian soil at any time from locations that suit our planning. Our heroic army has completed its preparations and plans. As I always say, we may come suddenly one night," Erdogan said on Monday.
Erdogan did not elaborate on Trump's response, but said Turkey would take care to avoid American casualties.
Trump told Erdogan that Turkish troops were not to enter the Syrian town of Manbij, which lies west of the Euphrates, according to a person briefed on the issue.
However, Erdogan has said Turkish forces will enter Manbij if the United States does not remove YPG fighters from the town and will also target the eastern side, where the YPG controls an area stretching more than 400 km (250 miles) along the border towards Iraq.
Manbij is near an area where Turkish and U.S. troops began joint patrols last month. That cooperation has also been complicated as Turkey has shelled Kurdish fighters to the east of the Euphrates.
The shelling prompted the United States to set up observation posts on the border between Kurdish-held northern Syria and Turkey.
Erdogan's comments appeared to be more conciliatory than those of his interior minister Suleyman Soylu, who earlier accused Washington of attempting to stymie Turkey's efforts against the YPG over the last two years.
"The United States thought it could deter us with the men it has nurtured," Soylu said during a visit to Pakistan, state-owned news agency Anadolu reported. "Now, they will try to hold us back east of the Euphrates. Turkey did not, and will not, allow that."
A spokesman for the Turkey-backed National Army rebel group in Syria said it would ignore a request from America for it not to participate in a campaign east of the Euphrates. "We have chosen to stand side by side with (Turkey)," Major Youssef Hamoud said.
Separately, Ankara has kept up regular air strikes against PKK militants based in Iraq's mountains. Baghdad summoned Turkey's ambassador on Friday after Ankara said it killed eight PKK fighters. But Turkish warplanes have since carried out further strikes.
(Additional reporting by Tuvan Gumrukcu and Ece Toksabay in Ankara and Ali Kucukgocmen in Istanbul; Writing by David Dolan; Editing by Richard Balmforth and by David Stamp)