(Reuters) - U.S. antitrust regulators are preparing to block Staples Inc's (>> Staples, Inc.) acquisition of smaller rival Office Depot Inc (>> Office Depot Inc), the New York Post reported.

The Federal Trade Commission (FTC) may again delay its decision on the deal, seeking more concessions from Staples, NY Post reported, citing two people familiar with the matter. (http://bit.ly/1XqZF9E)

The FTC is "ramping up for litigation," one person told NY Post. Two of the four FTC commissioners looking into the deal are believed to oppose it, the newspaper reported on Monday.

Staples shares rose 1.5 percent to $12.50 in light premarket trading, while Office Depot shares fell nearly 3 percent to $6.56.

Staples' first attempt to buy Office Depot was blocked by U.S. regulators in 1997.

The deal is also being investigated by EU regulators, who have raised concerns about possible price hikes.

Staples said in October that it had agreed with the FTC to extend the review period for the acquisition. The FTC is now expected to decide on the deal by Dec. 8.

Staples and Office Depot are in talks with wholesaler Essendant Inc (>> Essendant Inc) to transfer about $600 million of corporate contracts to satisfy U.S. antitrust concerns, the Wall Street Journal reported this month.

Staples, Office Depot and FTC representatives were not immediately available for comment.

(Reporting by Yashaswini Swamynathan in Bengaluru; Editing by Kirti Pandey)

Stocks treated in this article : Staples, Inc., Office Depot Inc, Essendant Inc