Two people have been killed and at least 14 others injured in two separate explosions on buses in the southwestern Chinese city of Kunming.

The cause of the explosions is not clear, but they are reported to have occurred within about an hour of each other during the peak Monday morning rush hour.

The blasts come as China ramps up security ahead of next month's showcase Olympic games.

Chinese authorities have warned that the games could be the target of terrorist attacks and have pledged an unprecedented security effort.

Among measures announced are the deployment of a 100,000-member anti-terrorist force, along with anti-aircraft missile batteries near to key locations.

Farmers killed

The Kunming explosions come two days after police in the same province, Yunan, opened fire and killed two rubber farmers in a clash that also saw 41 police officers injured.

The clash was sparked when police tried to arrest five people for allegedly attacking a local rubber company in a long-running dispute between farmers and the firm, state media said.

Chinese authorities have ordered local officials to keep a lid on grievances and try to resolve disputes to ensure a "harmonious social atmosphere" in the Olympics period.