Oil extended losses on Monday on worries of oversupply as the Organization of the Petroleum Exporting Countries (Opec) pumped at record levels in July, while weak China data stoked concerns about slower growth at the world's second largest oil consumer.

Saudi Arabia and other key members of Opec show no sign of wavering in their focus on defending market share instead of prices, as the group's oil output hit the highest monthly level in recent history in July, a Reuters survey showed.

The lack of a plan by Opec to make room for the return of more Iranian oil fuelled supply worries. Iran expects to raise output by 500,000 barrels per day (bpd) as soon as sanctions are lifted and by a million bpd within months, Oil Minister Bijan Zanganeh said in remarks broadcast on Sunday.

"The market seems to again focus on the supply situation ... one of the difficulties is that Iran may be coming back and there is no obvious sign that Opec will make room for them," Ric Spooner, chief market analyst at CMC Markets in Sydney said.

Brent fell 44 cents to $51.77 a barrel by 0213 GMT after touching an intraday low of $51.50, the lowest since Feb. 2. It is on its longest weekly losing streak since late 2014.

US crude fell 36 cents to $46.76 a barrel after hitting the lowest in four months at $46.35. Front-month prices lost 20.8 percent in July, the biggest monthly drop since October 2008.

Hedge funds and other speculators have slashed their bullish exposure to U.S. crude to the lowest in nearly five years, trade data showed on Friday, as local drillers continue to add rigs and pump at full throttle despite a global oil glut.

Growth at China's big manufacturing companies unexpectedly stalled in July as demand at home and abroad weakened, an official survey showed on Saturday, adding to worries from a recent slump in Chinese stock markets. – Reuters