A shipping container that fell off the back of a truck in southern Sydney has been moved, but traffic remains heavy.

The 13-metre-long container fell off the truck as it turned onto General Holmes Drive at Foreshore Road, Mascot, about 5.40am, blocking two city-bound lanes, a police spokeswoman said.

A Transport NSW spokesman said a crane moved the container off the road and all lanes were reopened before 8am.

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Two lanes had been closed for about two hours while crews worked to move the container, causing major delays for drivers.

Traffic remains heavy on the M5 East, Grand Parade and the Princes Highway.