By Benjamin Kang Lim

BEIJING, July 6 (Reuter) - Political tensions are rising in Tibet ahead of the 30th anniversary of the September 1 founding of the Chinese communist-controlled Tibet Autonomous Region, Western human-rights groups said on Thursday.

Police have beefed up security and launched a dragnet to find saboteurs who planted a bomb at a monument near roads leading to Qinghai and Sichuan provinces, the International Campaign for Tibet said from its Washington headquarters.

Police defused the bomb on June 25 in the restive Himalayan region, and offered a 10,000 yuan (US$1,200) reward for information leading to the capture of the independence activists presumed to have planted it, the group said.

Separately, Tibetan prisoner Lodroe Gyatso, 33, had six years added to his 15-year sentence for "instigating unrest in order to overthrow the government and split the motherland," the Tibet Information Network said from London.

Gyatso was found guilty of shouting slogans in Drapchi Prison in the region's capital, Lhasa, advocating Tibet's independence and praising the Dalai Lama, the exiled god-king of Tibetan Buddhism, the group quoted sources in Tibet as saying.

A Chinese official told Reuters by telephone from Lhasa he was aware of the bombing attempt but would not give details. A Lhasa city spokesman declined to comment on either report.

Security near the monument and elsewhere in Tibet has been increased ahead of the 30th anniversary of the founding of the Tibet Autonomous Region, which is kept under tight control by China's Communist Party and government.

A group of foreign tourists who tried to take photographs of the monument recently were detained briefly, International Campaign for Tibet said.

Chinese troops entered Tibet in 1950 and overthrew its Buddhist theocracy. Buddhist monks launched an anti-Chinese uprising in 1959, triggering a fierce crackdown that sent the Dalai Lama and thousands of followers into exile in India.

The nominally autonomous government was set up six years later. The Dalai Lama, who was awarded the 1989 Nobel Peace Prize, continues to campaign for Tibetan autonomy.

Beijing says Tibet has been part of China for centuries and that the region, one of the country's poorest, has flourished economically and socially under its rule.

Nonetheless, since 1987 Tibet has been rocked by numerous and often violent anti-Chinese demonstrations, usually led by Buddhists demanding independence and the Dalai Lama's return.

Tibet Information Network quoted Tibetan sources as saying prisoner Lodroe Gyatso had shouted slogans on March 4, calling for independence and praising the Dalai Lama.

He also handed out political leaftlets he had written, the sources said. A dance troupe member from northern Tibet, Lodroe Gyatso was jailed in 1993 for murdering a man in a fight. Tibet Information Network said he was threatened with execution after his March protest.

German parliamentarian Folder Neumann wrote China's ambassador Mei Zhaorong to Bonn to appeal for a stay of Lodroe Gyatso's execution, the rights group said.

Mei replied to Neumann that reports of a threatened execution were "unfounded and wrong," the group added.

The Germany embassy in Beijing said it had no information on the reported exchange of correspondence.