China says the United States is increasing its military presence in the Asia-Pacific region, which is becoming more "volatile".
It also says there has been a rise in operations directed against China.
The views were made in China's National Defence white paper, issued by the government.
The paper outlines the country's current views on security issues and gives an overview of its military forces.
In the document, released on Thursday, China gives a downbeat assessment of the regional security situation.
"Profound changes are taking shape in the Asia-Pacific strategic landscape. Relevant major powers are increasing their strategic investment," it says.
"International military competition remains fierce."
The document singles out the United States. According to China, it is reinforcing military alliances and getting more involved in regional affairs.
Beijing also says foreigners are now more suspicious of China - and have increased "interference and countering moves" against it.
Relations between China and the United States, particularly on military matters, have been strained over recent years.
That tension eased slightly following Chinese President Hu Jintao's state visit to the United States earlier this year.
But the potential for disagreement remains high.
"We admit that our military ties continue to face difficulties and challenges," said Col Geng Yansheng, spokesman for the Ministry of Defence, at a press conference to launch the white paper.
No issue threatens the relationship more than US support for Taiwan, a self-governing island off China's eastern coast that Beijing considers its own.
"The United States continues to sell weapons to Taiwan, severely impeding Sino-US relations," says the white paper.
Col Geng made it clear that the two countries must respect each other's core interests. For China, that includes Taiwan.
"China is willing to work with the US, based on respect, trust, equality and mutual benefits," he said.
The United States and some of China's neighbours occasionally express their concerns about the ultimate aim of Beijing's military modernisation.
But the white paper reiterates that no one has anything to fear.
It says China's armed forced, known as the People's Liberation Army, are there purely to defend the country.
China, it says, has a strategy of "attacking only after being attacked".