A 28-year-old primigravida at term presents to the labour ward in active labour. On admission, her cervix is 4 cm dilated, 80% effaced, and the fetal head is at –2 station. Contractions are occurring 2–3 times in 10 minutes, each lasting 40 seconds. A partograph is plotted. After 4 hours of labour, the cervix remains at 4 cm dilatation despite adequate contractions. The alert line on the partograph has been crossed. What is the most appropriate next step in management?