The US is closely monitoring China's expanding role in Africa, the latest secret US diplomatic cables released by Wikileaks reveal.
A cable from February quotes a senior US official in Nigeria's main city, Lagos, describing China as "aggressive and pernicious".
US diplomatic cables from Africa also reveal claims by oil giant Shell that it infiltrated Nigerian ministries.
Wikileaks has so far released more than 1,100 of 251,000 secret US cables.
The BBC's diplomatic correspondent Jonathan Marcus says the latest documents provide a fascinating insight into Washington's rivalry with Beijing in Africa.
China has massively expanded its economic ties to countries across Africa in recent years, sparking criticism from human rights groups, who accuse Beijing of helping some of Africa's worst governments stay in power.
China adopts a policy of not interfering in domestic politics, while Western countries sometimes make aid conditional on "good governance".
The cable, published by the Guardian newspaper, quotes Johnnie Carson, US Assistant Secretary for African Affairs, who had been meeting oil company representatives in Lagos.
He describes China as "a very aggressive and pernicious economic competitor with no morals".
"China is not in Africa for altruistic reasons," he says. "China is in Africa primarily for China."
He adds: "A secondary reason for China's presence is to secure votes in the United Nations from African countries."
He argues that China is not seen in Washington as a military or security threat at the moment. But he says there are, what he calls "tripwires" in Africa for the US when it comes to China.
"Have they signed military base agreements? Are they training armies? Have they developed intelligence operations? Once these areas start developing then the US will start worrying," he says.
"The United States will continue to push democracy and capitalism while Chinese authoritarian capitalism is politically challenging. The Chinese are dealing with the [Zimbabwean president] Mugabe's and [Sudanese president] Bashir's of the world, which is a contrarian political model."
Another US cable talks about China's military and intelligence support for the government of Kenya.
A Chinese enterprise is said to have won a contract to supply telephone monitoring equipment to Kenya after bribes were paid while on a trip to China.
The name of the individual concerned has been edited out.
Our diplomatic correspondent says the cable provides a case study of China's role in Africa.
Its influence in Kenya is said to have grown rapidly, with Chinese involvement in a host of infrastructure projects as well as collaboration with Kenya's National Security and Intelligence Service.
The secret cables also say that Shell's top executive in Nigeria at the time, Ann Pickard, told US diplomats that the oil company had good access to government information.
A cable dated 20 October 2009 outlines a conversation Ms Pickard had with the then US ambassador to Nigeria, Robin Renee Sanders.
When Ms Sanders asked the Shell executive about Chinese business interests in Nigeria, Ms Pickard told her that she knew that Nigerian officials had found Chinese offers not good enough.
"She said the [government of Nigeria] had forgotten that Shell had seconded people to all the relevant ministries and that Shell consequently had access to everything that was being done in those ministries," Ms Sanders reported.
The dispatches also show that Shell exchanged intelligence with the US about militant activity in the oil-rich Niger Delta, where activists say local people have suffered environmental damage because of the oil industry without reaping its economic rewards.
The BBC's Caroline Duffield in Lagos says the picture of Shell's tentacles reaching into government and accessing secret documents will shock ordinary Nigerians.
Environmentalists have long claimed the oil giant exerts a powerful political grip on Nigeria's government. Our correspondent says they will see these cables as evidence supporting that argument.
A Shell spokesman told the BBC the company could not comment on a leaked cable containing the views of a private conversation.
Wikileaks says it intends to release all the secret US cables in its possession, although it could take months to do so.
The move has been strongly condemned by the US and other countries.