Arctic sea ice has shrunk to the second smallest extent since satellite records began, US scientists have revealed.

The National Snow and Ice Data Center (NSIDC) says that the ice-covered area has fallen below its 2005 level, which was the second lowest on record.

Melting has occurred earlier in the year than usual, meaning that the iced area could become even smaller than last September, the lowest recorded.

Researchers say the Arctic is now at a climatic "tipping point".

"We could very well be in that quick slide downwards in terms of passing a tipping point," said Mark Serreze, a senior scientist at the Colorado-based NSIDC.

"It's tipping now. We're seeing it happen now," he told the Associated Press news agency.