US Vice President Mike Pence has escalated Washington's pressure campaign against Beijing by accusing China of "malign" efforts to undermine US President Donald Trump in upcoming midterm elections and criticising its tactics in the South China Sea.

Key points: Mr Pence said Beijing does not want Donald Trump to be US President

Mr Pence said Beijing does not want Donald Trump to be US President He slammed China for their treatment of Uighurs and bullying of countries over Taiwan

He slammed China for their treatment of Uighurs and bullying of countries over Taiwan Mr Pence said China has masterminded "wholesale theft of American technology"

In what was billed as a major policy address, Mr Pence sought to build on Mr Trump's speech at the United Nations last week in which he alleged that China was trying to interfere in US elections.

Neither Mr Trump nor Mr Pence provided hard evidence of Chinese meddling.

"China wants a different American president," Mr Pence said in a speech at the Hudson Institute think tank in Washington, adding the US will continue to expose Beijing's "malign influence and interference".

"As we speak, Beijing is employing a whole-of-government approach, using political, economic and military tools, as well as propaganda, to advance its influence and benefit its interests in the United States."

Chinese Foreign Ministry spokeswoman Hua Chunying said in a statement that Mr Pence had made "unwarranted accusations … and slandered China by claiming that China meddles in US internal affairs and elections".

China is committed to working with the United States for "non-conflict, non-confrontation, mutual respect and win-win cooperation", she said.

The allegations have raised questions about whether Mr Trump and his aides are trying to deflect attention from an investigation into his campaign's possible ties to Russian meddling in the 2016 presidential election, and also set China up for the blame if Republicans do poorly in next month's elections.

The vote will determine whether they keep control of Congress.

Sorry, this video has expired Trump accuses China of attempted election interference

Mr Pence said "as a senior career member of our intelligence community recently told me, what the Russians are doing pales in comparison to what China is doing across this country".

His speech marked a sharpened US approach toward China that goes beyond the bitter trade war between the world's two biggest economies and includes disputes about cyber activities, Taiwan, freedom of the seas and human rights.

Mr Trump has justified his tough trade policy against Beijing with accusations that China steals US intellectual property and limits access to its own market.

The two countries have imposed increasingly severe rounds of tariffs on each other.

Mr Pence said Chinese security agencies have masterminded the "wholesale theft of American technology", including military blueprints, and warned Washington would continue to take action.

He also urged Google to end development of its "Dragonfly" app that would make it easier to track internet searches and thus strengthen the Chinese Communist Party's censorship efforts.

Google declined to comment, except to reiterate that its China search engine project is "exploratory" and not close to launching.

Bloomberg Businessweek cited 17 unnamed intelligence and company sources as saying that Chinese spies had placed computer chips inside equipment used by about 30 companies, as well as multiple US government agencies, which would give Beijing secret access to internal networks.

But Apple and Amazon denied the report.

Mr Pence also said China had deployed anti-ship and anti-air missiles on islands it is building in the disputed South China Sea, despite promises not to militarise them.

USS Decatur (left) and a Chinese Destroyer (right) were pictured in the South China Sea. ( Supplied: US Navy via GCaptain )

He accused China of "reckless harassment" because of an incident on Sunday in which a Chinese naval vessel nearly collided with a US destroyer, the Decatur, near the Spratly islands.

"We will not be intimidated," Mr Pence said of the operation, the latest attempt to counter what Washington sees as Beijing's efforts to limit freedom of navigation in the strategic waters.

China's Defence Ministry said a Chinese naval ship had been sent to warn the US vessel to leave and that Beijing had irrefutable sovereignty in the area.

Mr Pence also joined other US officials in denouncing Beijing's crackdown on minority Uighur Muslims in the Xinjiang region.

Last month, a UN rights panel said it had received credible reports that up to one million ethnic Uighurs may be held in extra-legal detention in Xinjiang and US officials have warned of sanctions against China for human rights abuses.

China has said Xinjiang faces a serious threat from Islamist militants and separatists who plot attacks and stir up tensions.

Mr Pence accused China of using its economic power to bully smaller countries and said it had threatened the stability of the Taiwan Strait by pressuring three Latin American countries to sever ties with Taiwan and recognise Beijing.

China considers the self-ruled island a wayward province.

Reuters