China needs to move beyond a narrow focus on oil issues in South Sudan and help tackle that country's larger political disputes with Sudan, the outgoing U.S. special envoy to the two African states said on Wednesday.

Ambassador Princeton Lyman said he had worked closely with Chinese officials for more than two years, during which time South Sudan seceded from Sudan in 2011 to become the world's newest nation.

China is Sudan's biggest ally and is the largest investor in the oil industry there and in South Sudan - a position that Western diplomats say gives Beijing the best chance of defusing tension between Khartoum and Juba over sharing oil wealth and ending violence on both sides of their common border.

But Lyman said the disputes, which have shut down landlocked South Sudan's oil output, underscore the limits of staying aloof from political problems.

"They have weighed in very significantly on the oil issue. But what China doesn't like to do is to get involved in some of the underlying political problems that are keeping the oil from flowing," he told reporters in Washington.

"Without that stability and (with) the danger of conflict on the border, the chances of having a long-term productive oil sector is threatened, so they can't just concentrate on the oil and just pretend that the other things aren't bearing on it," he said.

Chinese Foreign Ministry spokesman Hong Lei, asked about Lyman's remarks, said China had consistently played an important role in promoting dialogue between the two Sudans.

"China's contributions have been well received by Sudan, South Sudan, African countries and regional bodies. We will continue to work with the international community for peace between the Sudans, stability and development, and play a proactive, constructive role," he told reporters in Beijing.

China has long held up as its foreign policy mantra non-interference in countries' internal affairs, a principle it first enunciated in 1954 - long before it was an economic power with interests around the globe.