ANKARA, Sept 21 (Reuters) - Turkish Prime Minister Tayyip Erdogan has cancelled a Sept. 22-25 trip to the United States where he was due to attend a U.N. General Assembly meeting, a source in his office said, citing an upcoming party congress and heavy work schedule.

Erdogan's ruling AK party will hold their annual congress on Sept. 30. Turkey's President Abdullah Gul had been scheduled to attend the U.N. assembly but cancelled due to an ear infection.

Erdogan, who was travelling in Gul's place, was expected to give a speech at the assembly in New York. The meeting is expected to discuss the crisis in Turkey's neighbour Syria among other issues. (Reporting by Pinar Aydinli; Writing by Jonathon Burch; Editing by Nick Tattersall and Angus MacSwan)