Turkish President Recep Tayyip Erdogan on Tuesday condemned comments by a key US envoy over the future of a US-allied Syrian Kurdish militia as a "grave mistake", as tensions flared over Washington's planned withdrawal from war-torn Syria. Erdogan's comments came shortly after US National Security Adviser John Bolton held talks in the Turkish capital with Erdogan's adviser Ibrahim Kalin, in a key meeting focusing on the surprise US decision to withdraw its troops from Syria.