BEIJING, Dec 30 (Reuter) - China has launched a manhunt across the restive region of Tibet, tightened airport security and offered a reward of $120,000 for the arrest of culprits who exploded a bomb in Lhasa last week, officials said on Monday.

The government had announced a reward of one million yuan ($120,000) for information leading to the arrest of those responsible for the bomb blast early on Christmas Day outside city government offices in the Tibetan capital, Lhasa, a senior official said.

``No one has yet been arrested and we are now mobilising and deploying our forces,'' Lhasa Vice Mayor Ou Yangxiang said in a telephone interview.

Notice of the reward had been published in the Lhasa Evening News on December 27 and it was being issued in other newspapers in the strategic Himalayan region, government officials said.

The Christmas Day bomb, the largest so far set off by anti-Chinese activists in restive Tibet, had resulted in no casualties but caused widespread damage, shattering windows within a radius of 100 metres (yards), officials said.

``Almost the whole of Lhasa heard it,'' said one Lhasa government official. ``It was a muffled sound, like whoong.''

Officials insisted there was little doubt that the blast was politically motivated and carried out by followers of the region's exiled god-king, the Dalai Lama.

The bomb exploded at night outside a government building and was therefore clearly political, said Zhou Kaifu, deputy director of the Tibet Public Security Bureau.

``This was done by the Dalai separatist camp,'' the government official said, adding that the attack bore the hallmarks of previous similar ``terrorist acts.'' He declined to give details although several smaller bombings have been reported in Lhasa.

``This is a terrorist act, aimed at destroying peace in our area,'' he said. ``This was a brazen act.''

Police had set up a special task force to search for the culprits and customs and immigration officials had stepped up security searches at Tibet's airport, he said.

The senior police official said the bomb was a home-made device made mainly from ammonium nitrate, a fertiliser.

``We are targeting Tibetan exiles returning from abroad and who have political connections,'' he said when asked about the focus of the manhunt.

``The bomb was made at a farm,'' he said. ``We are searching mainly in Lhasa, but are cooperating with other areas.''

China regularly blames followers of Tibet's exiled god-king, the Dalai Lama, for anti-Chinese unrest that erupts sporadically in the strategic Himalayan region that borders India.

The Dalai Lama, who won the Nobel Peace Prize in 1989 for his non-violent campaign to win autonomy for his homeland, says he wants self-government and freedom of worship in the deeply religious Buddhist region. He fled China in 1959 after an abortive uprising against Beijing rule.

Monks in the devout Buddhist region who have spearheaded anti-Chinese demonstrations and riots in the past were not regarded as suspects, the police official said.

Officials have vowed to retaliate, signalling a possible renewed crackdown on anti-Chinese unrest in the region.

``We should wage a tit-for-tat struggle against the Dalai clique's sabotage,'' local radio quoted Gyamco, vice chairman of the regional government as saying last week.

Several much smaller bombs have been been set off in Lhasa in the last two years, including one in 1995 that caused slight damage to a plaque donated by Beijing and another last March outside the headquarters of the Tibet regional government.