Oil has rounded out its best monthly gain in six years on Thursday, with the two global benchmarks for crude rising between 20 and 25 per cent in April, helped by a weaker dollar and bets that a supply glut would ease.

After the June-to-January selloff that halved oil prices from highs above $100 a barrel, Brent and US crude posted their strongest recovery this month, rising about $12 each, and hitting their highs for 2015.

Those were the best gains since May 2009, when crude prices were just recovering from the financial crisis, and backed oil bulls' contention that the supply-demand imbalance would level.

But some were skeptical of a smooth continuation to the rally, citing disappointing US economic growth in the first quarter and signs the Organization of the Petroleum Exporting Countries (Opec) had no intention of slowing output.

"It seems more likely that we will hit some speed bumps on this rally, given the unrelenting OPEC supply and the uneven U.S. economic data, which could cause dollar gyrations," said John Kilduff, partner at New York energy hedge fund Again Capital.

Brent settled up 85 cents at $66.69 a barrel, after reaching a 2015 peak of $66.93. It rose 21 per cent for April.

US crude rose $1.05 to settle at $59.63, hitting a 2015 high of $59.85 in post-settlement trading. It gained 25 per cent for the month.

The gains came as the dollar fell to a two-month low, making oil less expensive to holders of other currencies.

A Reuters survey of 32 analysts predicted Brent would average $60 in 2015, up 80 cents from last month's survey. US crude was seen at $54.40 versus $53.60 previously.

US oil inventories have been rising steadily for months but have begun to level off in recent weeks as domestic production eased and refinery demand picked up. On Wednesday, government data showed the first draw in five months at the US hub for crude storage in Cushing, Oklahoma.

Skeptics, however, point to Opec supply, which a Reuters survey showed hit more than two-year highs in April.

The dollar could also rebound to temper the oil rally. The currency came off its lows on Thursday after data showing US consumer spending rose in March and jobless claims hit 2000 lows.

"I think this is a rally to sell into," said Tamas Varga, oil analyst at London brokerage PVM Oil Associates. - Reuters