It was not immediately clear who was behind the bombing early on Wednesday, which came as tensions ran high in the Chinese region following a series of self-immolation protests by Tibetan monks.
"Several sources have confirmed a bomb blast but nothing is known about who carried it out or why," spokeswoman Kate Saunders from the International Campaign for Tibet told AFP by telephone.
"As it was carried out at 4:00 am, it is thought that no one was hurt," she added. The blast was in Tibet's Changdu county, known by Tibetans as Chamdo.
Tibetan news portal www.TibetExpress.net also reported the news, saying "Tibet?s independence" had been daubed in red on the damaged walls of the office building and "Free Tibet" fliers had been found at the scene.
"No one is accused or arrested in this connection so far but the entire road access leading to and from Chamdo had been completely cut off including closure of Karma monastery," the source told the website.
The author of the report, journalist Tenzin Wangchuk, said he had received the information from a local source.
"We got this information from a person who called us from that area and said he had seen the bombing. The information is authentic but we cannot identify him," he told AFP.
Nine Tibetan Buddhist monks and a nun have set themselves on fire in southwest China's Sichuan province, which has a large ethnic Tibetan population, since March.
At least six of them are thought to have died, according to rights groups.
Saunders said she recalled only two other bombings since 2008, when ethnic riots erupted. Two police stations were targeted in those attacks, she said.
"What we can expect now is a very serious police response, particularly in the current climate," she told AFP.
The last reported bombing was in Sichuan in March 2009, the 50th anniversary of a failed uprising against Chinese rule in the mountainous region.
The uprising led to the exile of the Dalai Lama, Tibetan Buddhism's most revered spiritual figure.
The previous year saw large riots in the Tibetan capital of Lhasa that spread to other areas in China inhabited by Tibetans, including Sichuan.
Since then, Tibetan Buddhist monks say they have been forced to undergo what the Chinese authorities call political re-education. Rights groups say some have suffered interrogation, torture and beatings.
China has ruled Tibet since 1951, a year after sending in troops to "liberate" the region.
Calls by AFP to government offices in Changdu county went unanswered Thursday night.
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