Controversial exploration of South Africa's Karoo region for shale gas might be worth the trouble if the work is subjected to strict controls, a national environment official said Wednesday.

"It does seem there is merit, from our own perspective as a department, on exceptional experimental work that would have to be undertaken under highly-controlled conditions," Environmental Affairs Director-General Nosipho Ngcaba told a press conference here.

But there would have to be a "clear understanding" of the technologies that would be used to extract the shale gas, Ngcaba was quoted by the Sapa news agency as saying.

She underscored the importance of making sure that fresh water sources were not contaminated by so-called fracking techniques, and noted that the operations "would require an enormous amount of water" in an arid area.

The South African government is under pressure from energy companies like Shell to allow exploration to proceed, but officials are waiting for the results of a enviromental-impact study that have been delayed.

Local landowners and environmentalists warn that fracking, the technique of hydraulically fracturing shale rock deep beneath the surface to release natural gas, would pollute underground water supplies.

A study by the Econometrix company that was paid for by Shell has estimated that there could be enough shale gas in the Karoo region to supply South Africa with energy for 400 years, at current rates of consumption.

The study also said that tens of thousands of jobs could be created.