Nor did it give a reason for the delay in the announcement, which came after rioting broke out in Tibet's capital Lhasa on March 14 and spilled over into nearby Chinese provinces populated by Tibetans.
The arrests "forcefully undermined Tibetan independence elements' arrogance and effectively maintained social stability in the area", Xinhua said.
The report could not be independently corroborated. Foreign reporters are barred from travelling to Tibet without permission.
The revelation cast a cloud over talks between China and the Dalai Lama's envoys -- the seventh round of dialogue since 2002 -- which had been due to open in Beijing on June 11.
The previous round was held in the southern Chinese city of Shenzhen on May 4 to discuss recent unrest in Tibet.
Xinhua said the suspects were rounded up last month and admitted carrying out the bombings in Changdu prefecture to "create a social impact and in coordination with the March 14 Lhasa incident".
Changdu, known in Tibetan as Chamdo or Qamdo, is the birthplace of Tibet's governor, Qiangba Puncog, who has been on the frontline of China's struggle against the Dalai Lama and his government-in-exile in India.
The suspects listened to overseas radio broadcasts over a long period and "bought into the Dalai clique's propaganda and incitement of ethnic separatism", Xinhua said.
The first incident involved five monks from Wese monastery who bombed the Mangkam county transformer on April 5, it said. The five were apprehended on May 13.
On April 8, four monks from Kebalong monastery fled after setting off a home-made bomb near the barracks of the paramilitary People's Armed Police, Xinhua said.
One of the suspects, Tashi Tsering, was taken into police custody on May 12, while three others are on the run.
In the third case, two men instructed four monks from the Kebalong monastery to bomb the home of a Tibetan farmer, Xinhua said without elaborating. The four monks were arrested on May 12.
Xinhua did not account for the other six in custody.
The Dalai Lama, who fled into exile in India in 1959 after an abortive uprising against Chinese rule, says he wants autonomy, not independence, for his homeland. But China is unconvinced and considers him a separatist.
China has accused followers of the Dalai Lama of instigating the March rioting -- the worst since 1989. The Dalai Lama denies the charge.
The unrest prompted anti-China protesters to disrupt the international leg of the Beijing Olympic torch relay and counter-protests by Chinese studying or living abroad.
Additional reporting by Guo Shipeng