A multi-vehicle accident is causing some traffic headaches in Sydney's west on Thursday afternoon.

Emergency services were called to Mamre Road, St Clair, at about 2.10pm following reports of a three vehicle crash.

Three patients required assessment by paramedics, with one person initially trapped inside a vehicle due to their injuries, a spokesman for NSW Ambulance said.

Two others who escaped with minor injuries have been taken to Westmead and Nepean hospitals in a stable condition.

The other person has since been taken to Westmead Hospital in serious condition.

Traffic in the area is heavy with Mamre Road closed in both directions between Luddenham Road and Erskine Park Road, and diversions are in place for both drivers and Route 779 buses.

Route 775, 776 and school buses are delayed by up to 30 minutes, and parents collecting children from a nearby childcare centre on Erskine Park Road have had to be escorted by police.

More to come.