India's benchmark indexes fell on Wednesday, led by drags in HDFC Bank, after the country's largest private lender flagged a hit to its asset quality post its merger with HDFC Ltd.

The Nifty 50 fell 0.64% to 20,002.75 points at 10:00 IST, poised for its worst session in almost a month. The S&P BSE Sensex dropped as much as 0.9% to 66,985.36 points, on track to see its worst day since early August.

The more domestically focussed mid-cap and small-cap firms were down 0.1% each.

HDFC Bank slipped as much as 3.3%, emerging as the Nifty 50's top loser after it said the completed merger with HDFC Ltd would affect some key financial ratios such as its net interest margin and non-performing assets.

"There was lot of uncertainty with respect to how the numbers of merged entity would be," Aneesh Srivastava, chief investment officer at Esquire Capital Investment Advisors said.

"With the recently concluded analyst meet, the management has tried to give some clarity," he added.

That in turn dragged the overall banking index, which dropped about 0.7% and was on track for its biggest intraday drop since late August.

Finance also saw its worst session since Aug 3, falling as much as 1.2%.

Other sectors such as IT and oil and gas added pressure on the benchmark, falling 0.4% and 0.6%, respectively.

Globally, markets expect the U.S Federal Reserve to opt for a pause at the conclusion of its two-day meeting later in the global day, albeit with a hawkish stance.

Among individual stocks, shares of textile-to-oil conglomerate Reliance Industries fell 1.9% on multiple block deals, according to LSE data.

(Reporting by Archishma Iyer in Bengaluru; Editing by Janane Venkatraman)