China flew a long-range bomber capable of carrying nuclear weapons over the South China Sea in recent days, according to US officials.

The move appears to be Beijing's way of flexing its military muscles in a worrying show of force that officials in Washington say is a message to President-elect Donald Trump, Fox News reported on Friday.

For the first time since Trump upended decades of diplomatic protocol and spoke on the phone with the leader of Taiwan, China flew aircraft over an area that includes disputed islands which it claims as its own.

The flight route corresponded to the so-called 'Nine-Dash line,' the demarcation boundary used by China to mark a number of islands that are also claimed by neighboring countries, including Taiwan, Vietnam, the Philippines, Indonesia, Malaysia, and Brunei.

A Chinese H-6 bomber is seen above as it flies over southern Japan in October, 2013. US officials say an H-6 was escorted by Chinese fighter jets in a recent flight over South China Sea

The last time Chinese planes flew along the route was March 2015, US officials told Fox News.

During the most recent flight, the Chinese H-6 bomber was at times escorted by fighter jets.

The United States has taken issue with what it perceives as China's increasingly aggressive stance in the contested South China Sea.

Pentagon officials are also increasingly concerned over the latest intelligence which indicates that China is making preparations to station advanced surface-to-air missiles on some of these islands.

This was the first time that an H-6 bomber flew over the contested South China Sea area in over a year, Pentagon officials said

The flight route corresponded to the so-called 'Nine-Dash line', an unofficial demarcation line which China uses to lay claim to a number of disputed islands in South China Sea

US satellites have detected components for China's version of the SA-21 surface-to-air missile at the southeast port city of Jieyang.

Fox News reported in February that China had deployed sophisticated SAM systems to Woody Island, a disputed island that is also claimed by Taiwan and Vietnam.

The US suspects that China is expanding its military reach into the South China Sea.

The latest military maneuvers are all the more disconcerting since they come against the backdrop of increasing tensions between Washington and Beijing.

Trump said on Thursday the United States needed to improve its relationship with China, which he criticized for its economic policies and failure to rein in North Korea.

US satellites have also detected components for China's version of the SA-21 surface-to-air missile at the southeast port city of Jieyang. An original Russian SA-21 is seen in this file photo

'One of the most important relationships we must improve, and we have to improve, is our relationship with China,' Trump told a rally in Iowa.

The United States and China are the world's two biggest economies.

'China is not a market economy,' he said. 'They haven't played by the rules, and I know it's time that they're going to start.'

Trump criticized China repeatedly during his presidential campaign and drew a diplomatic protest from Beijing last week after speaking by phone with President Tsai Ing-wen of Taiwan, which China considers a wayward province.

It was the first such top-level contact with Taiwan by a US president-elect or president since President Jimmy Carter adopted a 'one-China' policy in 1979, recognizing only the Beijing government.

President-elect Donald Trump (left) accepted a congratulatory phone call from the Taiwanese leader, Tsai Ing-Wen (right), on Friday, sparking controversy over a breach of protocol

Trump kept up his criticism of Beijing during the rally, which was part of a 'thank you tour' to express gratitude to states that helped him win an upset victory over Democratic presidential nominee Hillary Clinton last month.

'You have the massive theft of intellectual property, putting unfair taxes on our companies, not helping with the menace of North Korea like they should, and the at-will and massive devaluation of their currency and product dumping,' Trump said of China.