By Tom Perry and Laila Bassam BEIRUT (Reuters) - Kurdish-led forces in Syria seized a town from Islamic State on Tuesday after capturing a military base overnight, aided by U.S.-led air strikes in some of the most dramatic gains yet against the militants. The Kurdish march deep into the heart of Islamic State territory follows their capture of a town on the Turkish border last week, reversing the momentum of militants who had seized major towns in both Syria and Iraq last month. The Kurdish YPG-led forces had taken full control of Ain Issa, said YPG spokesman Redur Xelil.