A Chinese spy ship has been spotted off the Alaskan coast, with U.S. officials saying they believe it was there to observe a recent missile defense test, CNN reported Friday.

U.S. Navy Captain Scott Miller, a spokesperson for the North American Aerospace Defense Command, confirmed the presence of the ship and said it is believed to be a "communications" or "intelligence" vessel, according to the report.

The ship was operating legally in international waters and did not pose security concerns, but it is rare for China for operate such a vessel in that area, the report said.

The U.S. has no way of confirming the ship's intent, but its arrival in "the last few days" coincided with a Terminal High Altitude Area Defense (THAAD) missile defense system test off Alaska, reported CNN. The U.S. said on Tuesday that the test was successful.

China has long protested the decision to deploy THAAD to the Korean peninsula, saying it could be used to target Chinese missiles. But South Korean and U.S. officials say the system would exist only to intercept missiles launched by North Korea.

Read the full CNN report here.