The letter "was not in line with diplomatic and political courtesy. We will not forget this lack of respect. This is not a priority for us. But when the time comes we would like it to be known that we will take the necessary steps."
The extraordinary missive warned the Turkish leader not to be a "fool" over Turkish plans to start a military campaign in northern Syria.
"Don't be a tough guy, don't be a fool!", the letter, leaked from the White House, said.
In the 9 October letter, Mr Trump also told him he could make a “great deal” with General Mazloum Kobani, leader of the People’s Protection Units (YPG), the Kurdish-led militia that is allied with Washington in the fight against Isis.
He said he was offended by the suggestion that he would meet with Gen Mazloum Kobani. "You’re talking about a terror masterminded. This mastermind is talking to President Trump. This agreement is between Turkey and a state and not with a terrorist organisation."
Speaking at a press conference in a chamber in the ornate Dolmabahce Palace complex along the Strait of Bosphorus in Istanbul, Mr Erdogan warned that the war could start again if the YPG does not vacate the 30 kilometre buffer zone along its border"If the US can keep its promise, the issue of safe zone will be resolved." he said. "If these promises are not kept at the end of 120 hour-period our operation is going to continue where it left off."