SURUC, Turkey — A vanguard force of Iraqi peshmerga troops entered the embattled Syrian border town of Kobani from Turkey on Thursday, part of a larger group of 150 fighters that the Kurds hope will turn back an offensive by militants of the Islamic State group. The first group of Kurdish fighters crossed into Syria following heavy overnight clashes as Islamic State extremists unsuccessfully tried to capture the frontier post, the only gateway in and out of the town. Mustafa Bali, an activist based in the town, said the remaining troops will follow later in installments as a security precaution because the Islamic State fighters were targeting the border area.