Oil prices declined on Tuesday for a second day as rising U.S. output and a strengthening dollar sapped demand for crude, pushing Brent below $69 a barrel for the first time in six days.

Brent crude futures, the global benchmark, had declined 49 cents, or 0.7 percent, to $68.97 a barrel by 0522 GMT, after earlier dropping as low as $68.91, Reuters said.

Read alsoBloomberg: U.S. oil climbs toward Brent on weaker dollar and stockpile dropThe contract for March delivery settled down $1.06, or 1.5 percent, at $69.46 a barrel on Monday.

U.S. West Texas Intermediate crude futures dropped 70 cents, or 1.1 percent, to $64.86 a barrel. On Monday, they fell 58 cents, or 0.9 percent, to $65.56.

Prices are still heading for a fifth straight monthly gain.

"Markets remain fragile to the downside," said Stephen Innes, head of trading for Asia-Pacific at futures brokerage Oanda in Singapore, citing a jump in the number of rigs drilling for oil in the United States.

U.S. production is already on par with Saudi Arabia, the biggest producer in the Organization of the Petroleum Exporting Countries (OPEC). Only Russia produces more, averaging 10.98 million barrels per day (bpd) in 2017.

U.S. output has jumped more than 17 percent since mid-2016 and is expected to exceed 10 million bpd soon.

Drillers in the U.S. added 12 oil rigs for new production in the week to January 26, Baker Hughes reported on Friday.

The recent rally in oil prices had been fueled by the U.S. dollar's six straight weekly slides. The greenback is down 3 percent so far this month.

Oil is priced in the greenback, so a falling dollar can boost demand for crude from buyers using other currencies.

The dollar index had been below $90 since Jan. 24, falling below $89 on Friday. But the currency has rebounded since then to around $89.37, which has weighed on crude prices.

Investors are bracing "for the upcoming refinery maintenance season amid a strong USD," ANZ Research said in a note.

Crude prices may also be under pressure on expectations for U.S. inventories to rise for the first time in 11 weeks, according to a preliminary poll by Reuters on Monday.