(CNN)A Chinese spy ship has been sailing in international waters off the coast of Alaska for several days, having first arrived in the area shortly before the test of a US missile defense system, multiple US officials have told CNN.
US Navy Capt. Scott Miller, spokesperson for the North American Aerospace Defense Command, said the ship is believed to be a "communications" or "intelligence" vessel and confirmed it has been in the area for the last few days.
Miller would not speculate on the reason for the ship being there.
Two US military officials told CNN that the ship is operating approximately 100 miles off the Alaska coast in international waters.
The officials pointed out that the Chinese ship is operating legally and said the US does not have security concerns about the vessel.
US ships also regularly operate in international waters near China.
The officials said it is possible the ship was there to observe the recent successful Terminal High Altitude Area Defense (THAAD) missile defense system test off Alaska, but the US has no way of confirming that intent.
However, one official told CNN that it was rare for China to operate this type of ship in that area and that it was reasonable to assume it was there to observe the test.
China has long protested the US-South Korea decision to deploy THAAD to the Korean peninsula, saying that it could be used to target Chinese missiles.
South Korean and US officials say the system would be used to intercept missiles coming from North Korea.
The same official said that the US had observed similar Chinese vessels in recent days sailing off the coast of Guam and shadowing a joint US-Australia military exercise named Talisman Sabre. The official added this level of activity was unusual.