A truck has crashed and burst into flames on a major Sydney motorway, causing major delays.

The truck crashed on the M7, near the intersection of Elizabeth Dr, at Cecil Park about 4.50am on Thursday.

The M7 southbound lanes have been closed, sparking major delays for peak-hour commuters, with traffic reaching as far south as Hoxton Park.

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Transport NSW is urging motorists to use diversions and to allow extra travel time.

Police said no one was hurt in the crash, which is currently burning out of control.

More to come ...