A traffic jam more than 100 km long has formed in China just one week after gridlock led to tailbacks lasting 10 days northwest of Beijing.

'The motorway is blocked again,' a spokeswoman from the highways authority in Jining in Inner Mongolia told DPA Friday.

Traffic has mostly come to a halt for the past couple of days between Jining and Huai'an northwest of Beijing.

State television estimated the length of the traffic jam at 120 km.

The number of lorries involved was put at over 10,000, around half of them transporting coal.

Inner Mongolia is the main domestic source of coal, which is the country's largest source of energy.

'There is a lot of traffic and also some lanes have had to be closed due to construction work,' the spokeswoman said.

It was not known how long drivers would be stuck in the jam, the spokeswoman said.

Coal from Inner Mongolia is supplied to 14 provinces but rail and road capacity is inadequate.

Inner Mongolia last year replaced Shanxi province as China's largest coal producer.

The most recent traffic jam disappeared 10 days after it formed when road lanes were reopened and police stopped their normal checks on lorries.