Construction has started on a new bridge between China and North Korea, state media reported on Tuesday, as tensions simmer over the North's nuclear program.

The 804-metre long bridge spanning the Tumen river separating the two countries will cost 137 million yuan ($21.93 million), and is expected to open by 2016, China's official Xinhua news agency cited local authorities as saying.

China is North Korea's most important ally and its biggest trading partner, though analysts say it has grown increasingly impatient with its neighbour's ongoing nuclear test program.

The bridge will replace an older structure, built in 1938, which will be turned into a tourist attraction, Xinhua said. There are several other rail and road bridges linking the two countries.

Goods flowing across the border include minerals imported by China, while Beijing exports technology and luxury goods including pianos to Pyongyang.

China fears instability close to its border and is keen to avoid the collapse of North Korea's authoritarian regime. Earlier this month, Beijing defended Pyongyang against criticism voiced during a UN human rights review.