Beijing and Hanoi vowed to expand maritime and security cooperation on Thursday, further calming the waters of the South China Sea, where tensions have subsided since a transfer of power in the Philippines last year.

The agreements were among over a dozen sealed by President Xi Jinping and Vietnam's ruling party chief Nguyen Phu Trong in Beijing.

The meeting of the leaders and its timing was significant.

Trong's four-day visit, ending Sunday, is happening in close order with the visit of US Secretary of State John Kerry to Vietnam, starting Friday, and Japanese Prime Minister Shinzo Abe's arrival in the Southeast Asian nation on Jan 16. Observers said the visits reflect the complexity of relations among the four nations.

"We should strengthen strategic cooperation and keep boosting mutual trust, so as to lay a more solid political foundation for the settlement of maritime issues," Xi said at the meeting. "At the same time, we should push forward joint exploration and cooperation in the sea."

Xi, general secretary of the Communist Party of China Central Committee, suggested expansion of military exchanges and deepening of security cooperation. Trong said Hanoi will "expand maritime cooperation" with Beijing and seek results in areas like security, trade and youth communication. He affirmed that Hanoi takes relations with Beijing as "the top priority of long-term foreign policy".

This is the first visit to China for Trong, general secretary of the Central Committee of the Communist Party of Vietnam, since his re-election in January 2016. Xi and Trong last exchanged visits in 2015.

The two leaders witnessed the signing of 15 cooperation documents on Thursday, covering areas including national defense, financing, railways and Party cadre training.

In a rare arrangement for visiting foreign leaders, Xi and Trong also had an informa chat as they were having tea, a common culture in both countries, before starting dinner.

"The trip right before the visits of US and Japanese leaders also reflects the importance Hanoi attaches to Beijing," said Pan Jin'e, an expert on Vietnamese studies with the Chinese Academy of Social Sciences.

Pan said Vietnam favors conflict control in the South China Sea and is carefully watching progress on a code of conduct for the South China Sea. China said on Wednesday it will finish a framework with Southeast Asian nations by the middle of the year on the code.

Tensions in the South China Sea have abated since Philippine President Rodrigo Duterte has taken steps to mend relations with China since taking office in June.

"The visit comes at a good time for both sides to have a pragmatic discussion on the South China Sea. It is expected that Vietnam will be cooperative on the issue in the near future," Pan said.

Jia Duqiang, a senior Southeast Asian studies researcher at the academy, said, "China will also deepen ties, especially under the framework of the Belt and Road Initiative."

As for bilateral security cooperation, Jia said last year also brought a good sign: in October Chinese warships visited Vietnam's Cam Ranh Bay, a port of symbolic significance, for the first time.

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