The US has accused China of sending an uninvited surveillance vessel to international waters off Hawaii where a US-led naval exercise is being held.
China is among 22 countries taking part in the world's largest international naval drill, codenamed Rim of the Pacific (Rimpac), for the first time.
Despite this, the auxiliary general intelligence vessel, located at about 200 miles off the Exclusive Economic Zone (EEZ), lurked around, said the US Pacific Fleet.
Playing down any intelligence risk, US spokesperson Captain Darryn James said in a statement: "The US Pacific Fleet has been monitoring a Chinese Navy surveillance ship operating in the vicinity of Hawaii outside US territorial seas."
"US naval forces continually monitor all maritime activity in the Pacific, and we expect this ship will remain outside of US territorial seas and not operate in a manner that disrupts the ongoing Rim of the Pacific maritime exercise."
The spokesperson added that Beijing had dispatched a similar vessel in 2012 as well.
China, which has sent four ships to take part in the exercise, has defended its activities and insisted its vessels are free to operate in "waters outside of other countries' territorial waters".
The Chinese defence ministry, without referring specifically to the surveillance ship, said: "China respects the rights granted under international law to relevant littoral states, and hopes that relevant countries can respect the legal rights Chinese ships have."
China's participation in the exercise has been both welcomed and criticised in the US, with some arguing that it will help both countries to improve relations while critics say it will strengthen China's naval capability.
"This is not the first time we've been under surveillance while we're operating or exercising. However, one might say it's a bit novel when you participate in an exercise with participating units," said Per Rostad, commanding officer of the Royal Norwegian Navy's Fridtjof Nansen.