A herd of wild elephants on Indonesia's Sumatra has repeatedly outsmarted efforts to stop them stealing crops, wising up to attempts to chase them off with burning torches, a report said Monday.

The head of Way Kambas natural reserve in Lampung province, Hudiono, told the state-run Antara news agency that a herd of 25 to 30 elephants had been nightly roaming out of the reserve to raid crops since Thursday.

The elephants, previously only occasional visitors, have managed to clamber over earthen embankments built as an obstacle between the reserve and the fields by using their trunks to hold onto each other, he reportedly said.

An electrified wire fence was also no match for the canny beasts, he told the agency, saying they had felled it using tree trunks.

Even blazing torches no longer scare the night-time raiders. "This herd were once afraid of torches and could be herded out this way. But now they cannot be (herded out) this way again," Hudiono said.

"In the rainy season such as now, people usually plant alternative crops and perhaps this herd of elephants thinks this is instant food," he added. Such crops include peanuts, taro and yams.

Conflicts between wild animals and humans have long been on the rise on Sumatra, where jungle habitat is being increasingly taken over by encroaching settlements, plantations and industrial estates.

Only about 350 to 430 wild elephants remained on the island in 2003, according to environmental group WWF.