A third site, the Three Parallel Rivers of Yunnan Protected Areas, put on the World Heritage List last year, also needed to be examined, said the meeting of the UN's Educational, Scientific and Cultural Organisation in eastern Suzhou city on Tuesday.
UNESCO's World Heritage Committee is concerned about a series of 13 dam construction projects along the Nu river and their ecological impact, said Jing Sheng, a Chinese official working with the UN group.
The Forbidden City, seat of the Chinese imperial dynasty,
was placed on the World Heritage List in 1987 but its integrity is now threatened by modern buildings being constructed within the immediate vicinity.
The demolition of old traditional hutong (alley) neighbourhoods nearby has also caused concern.
"The committee requires the setting up of a buffer zone" around the site, Jing said.
For Potala, the home of Tibet's spiritual leader the Dalai Lama
before he fled to exile in India in 1959, UNESCO requested that the adjacent village of Shol, at the foot of the palace, be preserved.
"The village was formerly managed by Potala. It is the residence of the people who worked in the palace," said Jing.