A 30 y.o. parturient woman was taken to the maternity house with complaints of having acute, regular labour pains that last 25-30 seconds every 1,5-2 minutes. Labour activity began 6 hours ago. Uterus is in higher tonus, head of the fetus is above the opening into the small pelvis. Fetal heartbeat is 136/min. PV: cervical dilatation is 4 cm, uterine fauces is spasming at a height of parodynia. Head is level with opening into the small pelvis, it is being pushed off. What is the most probable diagnosis?