International benchmark Brent briefly clambered back above $13 a barrel in early Friday trade, trading at one point at $13.08, a rise of 21 cents.

Prices hovered around $13, slipping below that several times.

But the gains only partly made up for the mauling suffered late on Thursday when a torrent of institutional investor selling sent Brent prices plunging to $12.85.

This marked Brent's lowest point since March's Riyadh pact, when OPEC and non-OPEC producers clubbed together to pledge some 1.5 million barrels per day (bpd) of output cuts.

And it underlined market scepticism over producers' current efforts to cobble together another cuts package, following an agreement last week by Saudi Arabia, Venezuela and Mexico to trim a combined 450,000 bpd.

A recent whistle-stop of fellow Gulf producers by Saudi oil minister Ali al-Naimi managed to secure modestcommitments from Iran and Qatar, establishing cuts likely to total 800,000 bpd.

But analysts are calling for much bigger reductions to start chipping away at the enormous petroleum stockpile that is suffocating any price revival.

"The market's still looking terrible," said Nigel Saperia, head of oil trading at Bankers Trust International in London. "Oil producers really need to cut another million bpd."

Lawrence Goldstein of Petroleum Industry Research Foundation said the cuts might even have to be as high as 1.5 million bpd.

"The problem with OPEC right now is that it over-promises and under-delivers," he said, echoing widespread criticism that producers had failed to adhere to their March promises.

Less that one million bpd has actually been taken off world markets since then, with some putting reductions at just 750,000 bpd, half what was committed.

The producer indiscipline has allowed world petroleum stocks to balloon by as much as a million barrels daily, according to recent figures from theInternational Energy Agency.

And dire economic figures confirming that Japan is officially in recession piled further worries on producers, already suffering badly at the hands of Asia's economic slowdown.

Japan's first quarter GDP fell 1.3 per cent in real terms from the previous quarter, the second successive quarter Asia's dominant economy has shrunk.

"We are not going to see recovery out of Japan for the next year," warned Bronwyn Curtis, chief international economist at Nomura International in London.