A local worker can cost an employer 80 yuan a day while the Vietnamese are usually satisfied with 40 yuan a day, Mo says.

On Chongzuo City's 553 km border with Vietnam, only 260 border police are deployed, far from enough to curb the illegal immigration on 123 routes, says Li Zuozheng, chief of staff of Chongzuo's border police.

By March 20, Chongzuo's border police had stopped 420 illegal Vietnamese immigrants this year, of whom 175 were detained and investigated and 10 deported, Li says.

But no authority can tell the exact number of illegal immigrants from Vietnam working in the Pearl and Yangtze river deltas, he says.

The situation has worsened with the severe drought plaguing northern Vietnam, which has driven many farmers to seek work in China, said Fan Qi, head of Fangchenggang City's border police.

On a mountain road just 30 meters from China's official entry in Pingxiang City, groups of Vietnamese cross the border without any checks.

In just 10 minutes, 14 people in three groups came into China through the route while six people left carrying large bags of goods.

A spokesman with the regional police says a crackdown was launched in March to curb the immigration, with stricter identity checks and searches at customs and border check stations.

The border police of Fangchenggang City apprehended 56 illegal immigrants from Vietnam, as well as one Vietnamese and eight Chinese organizers on March 24, the spokesman said.

On April 4, the police seized 19 Vietnamese illegal immigrants in a railway station of regional capital Nanning City, he added.