Xinhua did not explain why the alleged incident was not reported earlier, and it did mention any casualties or damage in the explosion.
A man who answered the phone at the Gongjue county "public security bureau" confirmed that nine suspects had been detained.
Six were planting the bomb and three were shielding the suspects and covering up their crimes, he said.
The man refused to give his name because he said he was not authorised to talk to the media.
A woman at the Tibetan regional public security department said she was not sure about the case because it was still under investigation.
The attack was the first alleged bombing reported in Tibet since anti-China protests began on March 10 in the capital, Lhasa.
The protests in Tibet and other Tibetan areas of China later turned violent, with hundreds of shops torched and ethnic Han settlers coming under attack.
China says 22 people were killed in the riots, while Tibet's government-in-exile, based in the north Indian town of Dharamsala, has claimed at least 140 people died.
More than 1,000 protesters were detained.
Hu Jintao, the Chinese president, spoke on the issue for the first time on Saturday, during a meeting with Kevin Rudd, the Australian prime minister, on the sidelines of a regional economic forum in Hainan.
"Our conflict with the Dalai clique is not an ethnic problem, not a religious problem, nor a human rights problem," Xinhua quoted Hu as saying, referring to supporters of the Dalai Lama, the spiritual leader of Tibetan Buddhists.
|China has arrested more than 1,000 people in|
connection with the Tibetan protests [EPA]
"It is a problem either to safeguard national unification or to split the motherland."
On Friday, China labelled a group linked to the Tibetan government-in-exile a "terrorist organisation".
In recent weeks, massive demonstrations by pro-Tibet activists and other groups critical of China have accompanied the Olympic torch relay in London, Paris and San Francisco this month, stirring anger in China.
The Games are due to be held in Beijing in August.
Earlier this month, Beijing accused Tibetan independence forces of organising suicide squads of launching violent attacks against China.
Wu Heping, a spokesman for China's public security ministry, also claimed that searches of monasteries in Lhasa had uncovered a large cache of weapons.
Dalai Lama comments
The announcement of the Tibetan monks' arrests came on a day the Dalai Lama denied accusations yet again that he orchestrated the March protests to discredit China before the Olympic Games.
Speaking on Saturday, during a visit to the US city of Seattle, he reiterated his opposition to a boycott of the Olympics, but said China's record on human rights and freedom was "poor".
He said his message to China was: "We are not against you. And I'm not seeking separation."
The Dalai Lama has said he believess the Chinese people deserve to host the Olympics, despite the recent protests.