Xi's pledge comes as China is trying to show it is a responsible international player amid concern over its growing military might and territorial disputes in the Asia-Pacific region.
During a state visit to Washington on Friday, Xi agreed with US President Barack Obama that both countries would increase their "robust" peacekeeping commitments.
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They are among leaders from more than 50 countries who pledged some 40,000 troops and police, as well as equipment or training for UN peacekeeping missions during a UN summit chaired on Monday by Obama.
"China will join the new UN peacekeeping capability readiness system, and has thus decided to lead in setting up a permanent peacekeeping police squad and build a peacekeeping standby force of 8,000 troops," Xi said.
He also said China would provide $100 million in military assistance to the African Union in the next five years to support the establishment of an African standby force and to boost its capacity for crisis response.
At the later summit, Xi said part of a new 10-year, $1 billion China-UN peace and development fund set up by China would be used for peacekeeping operations.
China would give "favorable consideration" to future UN requests for more Chinese engineering, transport and medical staff, but operations' "exit strategies need to be timely formulated and executed", Xi said.
Obama, who held tense summit talks with Xi last week in Washington, shook his hand vigorously as he left the podium on Monday.
Operational experience
The US military told dozens of UN ambassadors and military advisers in New York in July that the UN needed rapid response forces, equipment and training.
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Washington pays more than 28 percent of the $8.2 billion UN peacekeeping budget, but Beijing says it contributes more personnel to peacekeeping missions than each of the other four permanent members of the UN Security Council: the United States, Russia, France and the United Kingdom.
The top five troop- and police-contributing countries are Bangladesh, Ethiopia, India, Pakistan and Rwanda, according to August data from the UN website.
China now provides around 3,000 of the more than 106,500 UN troops, police and advisers deployed by all countries, making it the ninth biggest contributor of peacekeeping personnel.
Its largest contingent is in South Sudan, where it has played a growing diplomatic role and is a major investor in the oil industry.
Experts have noted that China's expanding peacekeeping role in recent years parallels its desire to expand its military's capabilities farther abroad and could provide logistical and operational experience.
"They clearly want to create a more international armed force so they can operate in more challenging environments," said Douglas Paal, director of the Asia program at the Carnegie Endowment for International Peace.
During his earlier address to the General Assembly, Xi tried to allay concerns that his country's growing influence was a threat.
"We are committed to peaceful development. No matter how the international landscape may evolve and how strong China may become, China will never pursue hegemony, expansion, or sphere of influence," he said.
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