Donald Trump renewed his Twitter assault on OPEC, pushing the case for lower oil prices a week before the cartel meets to set production policy.

"Oil prices are too high, OPEC is at it again. Not good!," the US President tweeted on Wednesday morning. It's the second time he's used social media to rail against the Organisation of Petroleum Exporting Countries. He made similar comments in April.

The US is putting pressure on OPEC members to increase production as global benchmark prices flirt with $US80 a barrel, raising bowser prices in the months before the US votes in midterm elections that could decide which party controls Congress. A mix of new US sanctions on Iran, chaos in Venezuela and robust global growth has tightened the oil market. The US has lobbied Saudi Arabia and other members, arguing they need to raise output by 1 million barrels a day to keep prices in check, people told Bloomberg News earlier this month.

Saudi Arabia has floated several plans to fellow OPEC members as the kingdom seeks consensus before the meeting in Vienna next week. Russia, the leading country outside OPEC that's part of the current agreement, is also pushing for higher production. But the countries pushing for looser policy face staunch opposition from Iran and Venezuela, both under sanctions from Washington.