Pharmaceutical heavyweights Pfizer and Allergan have unveiled a record-setting $US160 billion ($A222.62 billion) mega-merger as US stocks finished modestly lower.

US giant Pfizer says its combination with the smaller Ireland-based Allergan would boost its offerings, permit $US2 billion in operational savings and enable Pfizer to enjoy a much lower tax rate in Ireland. If finalised, the deal will be the biggest pharmaceutical merger.

Allergan dropped 3.4 per cent while Pfizer was the biggest loser in the Dow, declining 2.6 per cent.

The Dow Jones Industrial Average fell 31.13 points (0.17 per cent) to 17,792.68 on Monday.

The broad-based S&P 500 dipped 2.58 (0.12 per cent) to 2,086.59, while the tech-rich Nasdaq Composite Index lost 2.44 (0.05 per cent) at 5,102.48.

Monday’s session “was very quiet with some market participants likely getting a head start on the Thanksgiving holiday,” Briefing.com said.

“With that in mind, it wouldn’t be surprising to see below average trading volume for the remainder of the week.”

US markets will be closed Thursday for the holiday, and hold shortened trading sessions Friday.

Tyson Foods surged 10.2 per cent after reporting an 88 per cent rise in fourth-quarter earnings to $258 million and predicting another record year in fiscal 2016.

GameStop fell 4.2 per cent after the video game retailer announced a drop in third-quarter sales and lowered its full-year earnings forecast.

Electronic Arts, a leading developer of video games, shed 4.8 per cent.

Amazon jumped 1.6 per cent as it unveiled additional holiday sales promotions on electronics, toys, kitchen appliances and other goods.