China's Vice Water Resources Minister Jiao Yong said Wednesday the massive mudslides in northwest China were caused by extreme climatic conditions, but he also warned of the dangers of illegal construction near rivers.

Zhouqu was designed to withstand water flow rates of 579 square meters per second, the national standard for mountainous counties like Zhouqu, Jiao said.

As a county constantly affected by drought, Zhouqu received less than 200 millimeters of annual rainfall on average in the past few years, he said.

Just before the mudslides happened, the county experienced 97 millimeters of rainfall in 40 minutes, according to Jiao.

"The situation was compounded by the county's geological conditions," he said.

Zhouqu has experienced several severe floods before, Jiao noted.

Zhou added that illegal constructions near the river have never caused geological disasters before.

Zhouqu is on the upper reaches of Bailong River.

But Jiao admitted there might be illegal constructions near rivers in the country's mountainous regions, as China is densely populated and local authorities are pursuing fast economic development.

Jiao said the Ministry of Water Resources was intensifying its management of rivers by clearing illegal constructions to curb floods.

At least 702 people were killed and 1,042 others are missing after Sunday's mudslides in Zhouqu.