"Starting from the first half of September this year, the border between China and North Korea and the Yunnan section of the Sino-burmese border, were shifted to the frontier border of the PLA," foreign ministry spokesman Kong Quan said.
"It has been decided by the relevant laws and regulations of the People's Republic of China to unify the mangement model of tha land borders over the whole country. The shift has been finished. It is a normal adjustment," he said.
Military officials told AFP on Monday that the troop movements were not considered a "military build-up," while acknowledging the military was now defending the area rather than the police.
"We have not got any news about troop build-ups on the Sino-DPRK (North Korea) border," said a defence ministry official surnamed Song, who said he had checked the situation.
Hong Kong's South China Morning Post on Sunday, citing an unidentified security source in China, said five divisions of Chinese People's Liberation Army (PLA) troops had been deployed in Yanbian since last month to deter a nuclear build-up in North Korea.
Large troop movements and new military barracks have also been seen in the border towns of Hanchun, Tumen, Kaishan, Sanhe and Baijing, while air force jets have frequently been seen flying over the capital Yanji, some 40 kilometres (25 miles) from the border, the report, filed from Seoul, said.
The source said troops were also in the area to help stem the flow of North Korean refugees fleeing to China to escape a long famine and recession in the hermitic state.
The policy mirrored the situation along China's border with Burma, where heroin smuggling from the notorious "golden triangle" poppy growing region has become rampant in recent years.
Top negotiators from the United States, North Korea, South Korea, Japan, China and Russia met in Beijing in late August to discuss the 11-month crisis over Pyongyang's suspected nuclear weapons programmes.