Iron ore and coal prices have continued to tumble as China was hit by a credit downgrade and fears demand for steel was falling.

The most heavily traded iron ore futures contract on the Dalian exchange was down another 3.4 per cent in afternoon trade, to be down more than 7 per cent for the week and 18 per cent off its recent peak a month ago.

Spot iron ore prices have fallen even more to about $US66 a tonne, its lowest level since May.

Another key component in steel making, coking coal, is down more than 10 per cent over the week, while futures trading in rebar steel — used in reinforced concrete and construction — has seen prices dip 5 per cent in recent days.

The latest bad news for the market was the big global credit ratings agency Standard and Poor's downgrading China's creditworthiness a notch to A+.

In announcing the downgrade, Standard and Poor's said China's economic and financial risks had increased after a prolonged period of strong credit growth.

Standard and Poor's downgrade follows a similar move by the other big ratings agencies Moody's and Fitch earlier this year.

Economy looks to be slowing again

An array of weaker data, including industrial production and infrastructure spending, as well as tighter credit conditions, dimmed sentiment in the commodity-dealing rooms throughout Asia.

Credit Suisse recently forecast the slow down would translate to a 70-million-tonne drop in demand in the months ahead, leaving spot iron ore prices averaging about $US55 a tonne through to Christmas.On top of that, Chinese authorities demanded production cuts of up to 50 per cent in key steel-producing centres to reduce air pollution ahead of winter.

Rio Tinto's announcement that it would return the $US2.5 billion ($3.2 billion) proceeds from its Coal and Allied sale to shareholders through a buyback mitigated the damage on the ASX.

Rio shares edged up 1 per cent in early afternoon trade, while BHP was flat, although over the week both are down more than 1 per cent.

• [Editor:Wang Linyan]