Another agreement created formal rules to govern use of a military crisis hotline, a move that aims to speed top-level communication.
Bonnie Glaser, a senior advisor for Asia at the Center for Strategic and International Studies, said the 2014 MOU applies anywhere U.S. and Chinese military naval and air assets might encounter each other.
"Of course, an aggressive operator could fail to implement them and cause an accident. There is no consequence outlined in the agreement for violating its terms," she said.
The 2014 MOU refers to military vessels' actions "at sea," implying territorial and international waters. CUES also refers to events "at sea."
"The CUES agreement addresses unplanned encounters at sea, regardless of any territorial claims," said Captain Jeff Davis, a Pentagon spokesman.
China's defense ministry declined to comment on the issue on Thursday. But China's Naval chief Wu Shengli told U.S. chief of naval operations Admiral John Richardson on Thursday that when the USS Lassen entered the disputed waters, Chinese navy ships warned the U.S. destroyer several times using CUES.
The warnings were ignored by the U.S. ship and the Chinese navy was "deeply concerned", Wu said, according to a Chinese navy statement on Friday.
Regional security analysts say China is ambiguous about precisely what it claims as territorial waters around the islands and reefs in the South China Sea.
While CUES offers guidelines for encounters between ships, the broader parameters are set by the United Nations' Convention on the Law of Sea (UNCLOS) - another area of Sino-U.S. dispute.
UNCLOS provides for countries to set 12 nautical miles from their coasts to treat as sovereign territory and a further 200 miles as exclusive economic zone (EEZ), giving them claim to any fishing or minerals on the seabed but full freedom of passage to international shipping.
One key difference between Washington and Beijing is the right of military vessels and planes to conduct surveillance in international waters seas, including an EEZ.
China has repeatedly objected to U.S. military operations off its coasts even if they are outside what it claims as its territory. Washington insists normal military activities are allowed in an EEZ under UNCLOS, including surveillance.