Five Chinese ships are operating off United States' Alaska coast, the Pentagon has said, in what is the first such instance of China's military vessels seen in the area. 

The five Chinese vessels comprise three Chinese combat ships, one amphibious landing ship and a replenishment vessel, The Wall Street Journal reported. 

The ships are said to be operating in international waters and heading towards the Aleutian Islands. 

The Chinese ships were spotted off the US coast even as US President Barack Obama was in Alaska for an event on climate change. 

However, the presence of the Chinese ships is being considered as 'non-threatening' by US officials.

"This would be a first in the vicinity of the Aleutian Islands. I don't think we'd characterise anything they're doing as threatening," a US defense official told WSJ. 

The Aleutian Islands are divided under US and Russia's control. 

"We respect the freedom of all nations to operate military vessels in international waters in accordance with international law," US defence department spokesman Bill Urban told BBC.

Though not seen as a threat, the operation of the Chinese vessels so close from Alaska is likely to further raise tensions between US and China over the latter's military activities, weeks ahead of Chinese President Xi Jinping's visit to the United States.