(BBC) - Turkish President Recep Tayyip Erdogan put US President Donald Trump's letter "in the bin", the BBC has been told.
In the letter dated 9 October, and sent after US troops were pulled out of Syria, Mr Trump told Mr Erdogan: "Don't be a tough guy. Don't be a fool!"
President Trump was urging Turkey not to launch a military offensive against Kurdish-led forces in northern Syria, but Mr Erdogan ignored this request.
US Vice President Mike Pence is now in Ankara to push for a ceasefire.
The US has faced intense criticism for the withdrawal of troops, which critics say gave Turkey the green light to launch the military attack.
Turkey says the YPG is an extension of the Kurdistan Workers' Party (PKK), a rebel group fighting for Kurdish autonomy in the region.