A 20 year old primigravida is admitted with full term pregnancy and labour pain. At 4 am she goes into active phase of labour with 4 cm cervical dilatation. Membranes rupture during p/V examinatin showing clear liquor. A repeat PN examination after 4 hours of good uterine contraction reveals a cervical dilatation of 5 cm. What should be the next step in management :