The aftershock, measuring 6.1 on the Richter scale, struck the county of Qingchuan at 5:49 p.m. local time (9:49 GMT), killing two people and injuring 29 others, the news agency said. More than 2,700 houses in the county were also destroyed.
In the neighboring Gansu province, in northwest China, one person died and six were seriously injured.
The shock's epicenter was 50 km (30 miles) northwest of the town of Guangyuan.
The aftershock occurred just hours after the Olympic torch relay was held in the Sichuan capital of Chengdu. This was the last stop before the start of the Beijing Games on Friday.
The latest aftershock comes less than three months after the province was hit by China's worst earthquake in three decades, measuring 8.0 on the Richter scale. At least 70,000 people were killed and 10 million displaced in the May 12 quake.
