A Chinese official dismissed reports that a bizarre text message had warned residents of Kunming to avoid buses hours before two bomb blasts killed two passengers in Monday's rush hour, state media said.

The attack, which came during a nationwide security clampdown ahead of next month's Beijing Olympics, also wounded 14 people in the city, capital of the mountainous southwestern province of Yunnan.

Local media reported that police were investigating a text message allegedly received by residents hours before the blast, warning them not to take buses on Monday morning.

"The general mobilisation of ants. (I) hope citizens receiving this message will not take bus lines 54, 64 and 84 tomorrow morning," the Southern Metropolitan Daily quoted the message as saying.

The explosions, which came within an hour of each other, hit two line 54 buses close to each other, blowing holes in the side. In both cases, ammonium nitrate was wrapped under the seats.

Kunming vice mayor Du Min dismissed the message as a fabrication.

"In fact, there was no such message," Xinhua news agency quoted Du as saying on Tuesday.

One of the injured passengers was in critical condition, while most of the others suffered shattered eardrums, state media reported earlier.

Police had started roadside checks in Kunming and stepped up security across the province. A team of experts had also arrived from Beijing.

Police had also offered a reward of 100,000 yuan (7,318 pounds) for any information leading to solution of the case, Xinhua news agency said.

The attack happened less than three weeks before the Aug 8-24 Beijing Games, which China has warned could be a target of terror attacks.

Chinese Foreign Ministry spokesman Liu Jianchao condemned the blasts, but said there had been no evidence to link them with terrorists.

"There has been no evidence so far to show that the incident is related to the upcoming Beijing Olympics," Liu said at a news conference.

China has occasionally had bus explosions staged by disgruntled farmers or laid-off workers wanting to air grievances over poverty, demolitions or corruption.