China and the United States have signed annexes to two agreements regarding major military operations and naval and air force encounters.

Beijing confirmed the news - which comes amid President Xi Jinping's state visit to the US - on Thursday, saying the annexes had been signed by defense officials from both countries on Sept 18.

Observers said the approval signifies major policy progress and consensus to avoid miscalculation in the Western Pacific at a time when the two militaries are increasingly overlapping there, sometimes triggering tension accidentally.

Wu Qian, newly appointed Ministry of National Defense spokesman, told a scheduled news conference in Beijing the two annexes - "notification of military crisis" and "encounters in the air" - had been signed as part of two mutual trust-building mechanisms between the two countries.

The development marks new progress in building the mechanisms - a reporting system on major military operations and a code of safe conduct on naval and air force encounters, Wu said.

The "encounters in the air" annex clarifies rules, including those to ensure safe flying, exercising fundamental liaison and communication, announcing designated danger zones or warning zones, and coordinating efforts at the scene of emergencies.

The first annex is aimed at regulating mutual notification of a military crisis by using the telephone hotline between the two defense ministries, Wu said.

Zhang Junshe, a senior researcher at the PLA Naval Military Studies Research Institute, said the "encounters in the air" annex will be extremely useful, as it will serve as a set of rules to ensure that aerial encounters by the two militaries over the ocean are guided and regulated.

"Strategic mutual trust between the two militaries will be strengthened, and there will be less chance of incidents or conflicts taking place," Zhang said. "Miscalculations or misunderstandings can be avoided as much as possible."

China and the US agreed on the two trust-building mechanisms in November by signing a memorandum of understanding. They were proposed by President Xi Jinping during a meeting with his US counterpart Barack Obama in 2013.

Wu, a military diplomat who once served as deputy defense attache at the Chinese embassy in the US, said the Chinese and US defense ministries have communicated and cooperated closely on establishing the mechanisms over the past two years.

Wang Fan, vice-president of China Foreign Affairs University, said that in the past two years the two countries "have deepened their understanding of the real strategic intentions of each other".

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