BEIJING (Reuters) - A government office in an ethnic Tibetan part of China's southwestern province of Sichuan was hit by a bomb early on Monday but no casualties were reported, the official Xinhua news agency said.

The building, in the Ganzi region of Sichuan, was hit by a bomb thrown by terrorists, Xinhua quoted unidentified sources as saying.

An investigation team has been set up to look into the attack, it said in a brief report, without elaborating.

Rioting broke out in Tibet's capital Lhasa on March 14 last year after days of protests against Chinese rule by Buddhist monks, killing 19 people and sparking waves of protests across Tibetan areas.

Groups of Tibetan exiles say more than 200 people died in the crackdown.

A year later, a tight web of troops and police across Tibetan areas has apparently helped deter any flare-up of unrest other than small, isolated incidents.

Beijing has promised the region will be calm this year and President Hu Jintao called for a Great Wall of stability there.

Tibet and ethnic Tibetan areas of surrounding provinces have a heavy military presence and are strictly off limits to foreign journalists and tourists. Armed police manning road-blocks have turned back would-be visitors.

A trickle of isolated protests in recent weeks, including a monk who set himself on fire at the Kirti monastery in western Sichuan, suggest lingering discontent.

Many Tibetans did not celebrate their New Year in February, in silent protest and mourning for those who died last year.

(Reporting by Ben Blanchard; Editing by Paul Tait)