(Corrects headline to show oil price was flat)
By Amanda Cooper
LONDON, June 21 (Reuters) - Oil steadied on Wednesday, paring earlier losses, but was set for its largest price slide in the first half of any year for the past two decades, as investors discounted evidence of strong compliance by major producers with a deal to cut global output.
August Brent crude futures were flat at $46.02 a barrel by 1107 GMT, having fallen earlier to seven-month lows.
U.S. crude futures for August delivery were up 4 cents at $43.55, having hit their lowest since September on Tuesday.
So far this year, oil has lost 20 percent in value, its worst performance for the first six months of the year since 1997.
Compliance with an agreement by the Organization of the Petroleum Exporting Countries and other producers to cut output by 1.8 million barrels per day from January reached its highest in May since the curbs were agreed last year.
"The slide in oil prices seems to be unstoppable," said Julius Baer commodities research analyst Carsten Menke.
"The supply deal’s effectiveness is increasingly questioned. We believe that downside risks to oil prices from a (disorderly) and early unwinding have risen ... we still see prices trading sideways, spending more time in the high 40s than the low 50s as growing shale output and stagnant western-world oil demand undermine the Middle East's restriction efforts."
Data from the American Petroleum Institute on Tuesday showed U.S. crude stockpiles last week had dropped more than forecast. Gasoline and distillate inventories rose.
A government report on inventories is due at 10:30 a.m. EDT (1430 GMT) on Wednesday and the official figures often differ sharply from those of the industry group.
OPEC and non-OPEC oil producers' compliance with the output deal reached 106 percent in May, a source familiar with the matter said on Tuesday. That means they cut output by more than they were required to do.
OPEC compliance with the curbs was 108 percent, while non-OPEC compliance was 100 percent, the source said. Another source confirmed compliance by all producers in May was 106 percent.
While compliance is high, it is what went on before the production cut that counts, BMI Research said in a note.
"A number of producers - notably Iraq, Saudi Arabia and Russia - aggressively ramped up output in the run-up to the deal, fast-tracking projects, expanding drilling programmes and deploying spare capacity," BMI said.
Additional reporting by Aaron Sheldrick in TOKYO; Editing by Dale Hudson and Adrian Croft