The United States is watching growing cooperation between Russia and China in the Arctic closely and some of their recent military collaboration in the region sends "concerning signals", the U.S. Arctic ambassador said.

Russia and China have stepped up military cooperation in the Arctic while deepening overall ties in recent years that include China supplying Moscow with dual-use goods despite Western sanctions on Russia over the war in Ukraine.

Russia and the United States are among eight countries with territory in the resource-rich Arctic. China calls itself a "near-Arctic" state and wants to create a "Polar Silk Road" in the Arctic, a new shipping route as the polar ice sheet recedes with rising temperatures.

Michael Sfraga, the United States' first ambassador-at-large for Arctic affairs, said the "frequency and the complexity" of recent military cooperation between Moscow and Beijing in the region sent "concerning signals".

"The fact that they are working together in the Arctic has our attention," Sfraga, who was sworn in last month, told Reuters in a telephone interview from Alaska. "We are being both vigilant and diligent about this. We're watching very closely this evolution of their activity."

"It raises our radar, literally and figuratively," he added.

Sfraga cited a joint run by Russian and Chinese bomber planes off the coast of Alaska in July, and Chinese and Russian coast guard ships sailing together through the Bering Strait in October.

He said these activities had been conducted in international waters, in line with international law, but the fact that the bombers flew off the coast of Alaska had raised concerns for U.S. security.

"We do need to think about security, heighten our own alliances, our own mutual defences," Sfraga said. "Alaska, the North American Arctic, is NATO's western flank and so we need to think about the Arctic that way."

The activity was also a concern for U.S. allies as the Bering Strait and the Bering Sea give access to the North Pacific and South Pacific, he said.

The Pentagon said in a report released in July that the growing alignment between Russia and China in the Arctic was "a concern".

China and Russia are trying to develop Arctic shipping routes as Moscow seeks to deliver more oil and gas to China amid Western sanctions. Beijing is seeking an alternative shipping route to reduce its dependence on the Strait of Malacca.

The Arctic also holds fossil fuels and minerals beneath the land and the seabed that could become more accessible with global warming.