A woman is 40 weeks pregnant. The fetus is in the longitudinal lie and cephalic presentation. Pelvic size: 26-29-31-20. Expected weight of the fetus is 4800 gram. The labor contractions have been lasting for 12 hours, within the last 2 hours they were extremely painful. The parturient woman is anxious. The waters broke 4 hours ago. On external examination the contraction ring is located 2 finger widths above the navel, Henkel-Vasten sign is positive. Fetal heart rate is 160/min., muffled. On internal examination the uterine cervix is fully open, the head is engaged and pressed to the entrance into the lesser pelvis. What is the most likely diagnosis?