Work conditions at Zambia's Chinese-owned copper mines have improved, but the Asian giant's labour record is still clouding its sizeable investments into the country, Human Rights Watch said Friday.

The government-owned China Non-Ferrous Metals Mining Corporation (CNMC) has poured cash into Zambia and created jobs but was accused of poor practices in a damning report released by the watchdog last year.

"It has incredibly important potential for Zambia but a lot of this goodwill is being lost through its labour record," said Africa researcher Matt Wells about the state-run corporation, which has four subsidiaries in Zambia.

"The Chinese investment has huge potential but it has to come with respect for labour rights and most importantly that fall on the local governments to enforce," he added.

China is Zambia's top foreign investor but labour relations have been controversial, highlighted by the killing of a Chinese mine boss in August during a riot over wages at a privately owned coal mine.

Human Rights Watch slammed the four Chinese state-owned firms last year for ignoring labour protections, demanding up to 18 hours of work a day, and flouting health and safety rules.

"There have been real improvements but it still lags behind on these key areas of safety in comparison to other companies" investing in Zambia, said Wells.

China has invested more than two billion dollars in Zambia and created 50,000 jobs, with trade between the two reaching $3.4 billion last year up from $2.85 billion in 2010, Beijing's ambassador to Lusaka Zhou Yuxiao said this month.

Zambia enjoyed a trade surplus of $2.17 billion which was nearly the equivalent of its current foreign exchange reserve, he said.