Syrian Kurdish forces say Turkey is failing to abide by the terms of a US-brokered cease-fire. They say Ankara is refusing to lift a siege it imposed on a key border town in northeastern Syria 30 hours after the truce went into effect.<br />'' <br />'' The Syrian Democratic Forces today called upon U.S. Vice President Mike Pence, who negotiated the deal with Turkish President Recep Tayyip Erdogan, to take responsibility for enforcing the five-day cease-fire.<br />'' <br />'' The cease-fire got off to a rocky start, with sporadic fighting and shelling around Ras al-Ayn yesterday. The border town is a test for the deal in which Turkey asks that Kurdish fighters vacate the frontier zone.<br />'' <br />'' The Britain-based Syrian Observatory for Human Rights said that Turkey-backed Syrian fighters have prevented a medical convoy from reaching Ras al-Ayn since yesterday.<br />''