“Such a move could be defined as a serious provocation,” said the person, who requested anonymity because of the sensitivity of the issue.

In the 90-minute phone conversation, Esper raised concerns about the Chinese military’s “destabilising activities in the vicinity of Taiwan and the South China Sea” and called on China to abide by international rules.

Meanwhile Wei stated China’s positions on the issues and the US “stigmatisation” of China, and urged the US to better control maritime risks and warning against “dangerous moves” that might escalate the tensions, according to accounts by both sides.

The US rejected China’s sovereign rights in the South China Sea last month and conducted “dual carrier operations” in the waters, fuelling concerns of a hot war between the two countries, whose relationship has plunged to its lowest level in four decades.

The South China Sea Strategic Situation Probing Initiative, a Beijing-based think tank, indicated a sharp rise in the number of US military reconnaissance aircraft over the South China Sea in the past two months, sometimes less than 100km (62 miles) from the Guangdong coastline and including a sortie by an E-8C reconnaissance aircraft close to Guangdong in a rare night operation on Wednesday.

Li Jie, a Beijing-based naval expert, said the night operation by the E-8C close to the Chinese coast was “highly sensitive” as “it signals the possibility of a night sneak raid by the US military against China, therefore China should require the US to clarify”.

“The talk could enable both sides to lay out respective bottom lines and positions and could help prevent sparking an accidental clash,” Li said, adding there was an urgent need for dialogue.

Zhang Junshe, a researcher with the PLA Naval Research Academy, said in a commentary in the PLA Daily on Friday that US bullies and hegemonic activities had become “the disturber, destroyer and spoiler” to the peace and stability of the region. Zhang said the US must stop military provocation in the waters.

However, in a conciliatory note, both sides said there was a need to communicate to contain risk. Xinhua said the two sides exchanged views on bilateral ties and military relations, as well as “the bilateral military exchange for the next phase”.

The Pentagon said the two agreed on the importance of “maintaining open channels of communication and developing the systems necessary for crisis communication and risk reduction”.

On Wednesday, Esper told the Aspen Security Forum that China had become “more aggressive” during the Covid-19 pandemic and was “a pacing threat” for the US military.

He said the US had to be “much more vigorous in all domains”.

Chinese military plans live-fire drills amid rising Taiwan tensions

“I don’t see China right now as an inevitable threat, that we are going to have a fight with them,” Esper told the forum. “But we do have to compete.”

The confrontation between the two powers has led to worries over the possibility of a hot war, especially over the South China Sea. Observers said the communication between the defence chiefs had now served as another channel for Beijing and Washington to maintain engagement amid their highly contentious rivalries.

Song Zhongping, a military commentator based in Hong Kong, said the call indicated that both sides had reached a consensus to avoid miscalculation amid the heightened tensions.

“Both sides need to identify the crux of the current tensions and figure out a final solution,” he said.

“Both China and the US must prepare for two eventualities. They are now preparing for military fights, and there is no need to evade it. But on the other hand, both sides need diplomatic ways to test each other’s red lines and understand each other’s real political and military intentions.

“The divergence is grave, it’s hard to bridge the rift in many issues, but the two sides can always find an intersection – managing differences, avoiding miscalculation and reducing conflicts.”