[Editor's note: This story was updated at 11:51 a.m. CT March 21.]

Oil prices on March 21 fell close to its lowest in a week as the market discounted the latest talk by OPEC that it would extend output cuts beyond June.

That decline also came ahead of the release of weekly U.S. crude inventory data later March 21 and on March 22 that is expected to show a crude stock build of 2.6 million barrels (MMbbl), according to a Reuters poll.

Brent futures for May delivery were down 53 cents, or 1%, at $51.09/bbl by 10:36 a.m. CT (15:36 GMT). U.S. West Texas Intermediate (WTI) crude was down 68 cents, or 1.4%, at $47.54/bbl on the last day the April contract is the front-month, its lowest level since March 14.

RELATED: OPEC Leaning Toward Oil Cut Extension, Sources Say

Sources within OPEC have indicated that its members increasingly favor extended production cuts but want the backing of non-OPEC oil producers, such as Russia, which have yet to deliver fully on existing reductions.

"OPEC is sticking with a plan that has not worked," said Phil Davis, managing partner at PSW Investments in Woodland Park, N.J., noting they are not offering more cuts and no new countries are joining in the cuts.

The group and some non-OPEC producers agreed to curb production from Jan. 1 by 1.8 MMbbl/d for six months to drain crude from record stockpiles. But inventories remain stubbornly large.

"We think it is very unlikely that Russia will actively take part in any extension of the production cuts that goes beyond paying lip service to the agreement," Commerzbank said in a note, adding that it would be premature for investors to "pin their hopes" on an extension.

Commerzbank said that OPEC cuts would need to last into the fourth quarter to achieve the group's goal of reducing record oil stockpiles in industrialized nations to their five-year average.

Investors were awaiting U.S. inventories data expected to reveal rising stocks after a surprise drop in the week to March 10, with data from the American Petroleum Institute at 3:30 p.m. CT March 21 and the U.S. Energy Information Administration at 9:30 a.m. CT March 22.

The WTI delivery hub in Cushing, Okla., could be a particular focus in API data.

Stocks at Cushing rose in the week to March 10, helping to widen the premium for Brent over WTI. That gap now stands at around $2.82 for May delivery, its highest since the end of January.

"Another increase would be generally bearish for WTI-related spreads," said Tamas Varga, analyst at London broker PVM Oil Associates.