Five Chinese naval ships are currently positioned in the Bering Sea off the coast of Alaska, US officials confirm.
It is believed to be the first time Chinese military vessels have been seen operating in the area.
Officials say they have been monitoring the ships' activities, but said they were operating in international waters.
In recent years Beijing has taken a more assertive stance on maritime territorial disputes with Japan and South East Asian nations.
US defence officials have spotted three Chinese combat ships, a supply vessel and an amphibious ship moving toward the Aleutian Islands which is split between Russian and US control, according to the Wall Street Journal.
They were seen not far from where US President Barack Obama is visiting as part of his three-day tour of Alaska to raise awareness of the effects of climate change on the state.
"We are aware of the five People's Liberation Army Navy (PLAN) ships in the Bering Sea. This is the first time we have observed PLAN ships in the Bering Sea," US defence department spokesman Bill Urban told the BBC on Wednesday.
"We respect the freedom of all nations to operate military vessels in international waters in accordance with international law," he said.
Another official told the Wall Street Journal the defence department did not "characterise anything they're doing as threatening".
Peter Dutton, director of the China Maritime Studies Institute, described it as a big advancement in the way Chinese ships operate.
But, he tells the BBC, "it's not a surprise in a sense that the Chinese have been continually expanding their presence in Eurasia".
"They have conducted exercises with Russia in the Mediterranean and in the Sea of Japan... they have interest in the northern sea route so to see them off the coast of Alaska is the next evolutionary step along these lines."
Patrick Cronin, of the Center for a New American Security, agrees, saying: "It's part of a pattern over the last six years or so of China getting tougher in maritime space."
China wants to rewrite international maritime laws so that they are more favourable to its own interests, particularly in the South China Sea where rival countries are wrangling over territory, Mr Cronin adds.
China this year boosted its defence spending in a bid to modernise its forces, including developing stealth fighters and anti-satellite missiles, a move that has unnerved the US and its allies in the Asia-Pacific region.
President Xi Jinping will on Thursday preside over an enormous military parade in Beijing in commemoration of China's World War Two victory over Japan 70 years ago.