Stock futures declined on Tuesday morning as U.K. Prime Minister Theresa May outlined long-awaited plans for Britain's exit from the European Union. 

S&P 500 futures were down 0.4%, Dow Jones Industrial Average futures fell 0.3%, and Nasdaq futures slid 0.4%. 

The U.K. hopes to gain the greatest access to the European Union without being a member of it, May said in a highly anticipated speech on the country's plans for removal from the bloc. May also said she seeks to avoid a "cliff edge" that could prove disruptive and instead intends for the 'Brexit' to come in phases. The eventual deal would be voted upon in both Houses of Parliament. The U.K. voted to exit the regional bloc in June after four decades of participation. 

The FTSE 100 in London declined 0.9%, while the British pound rallied more than 2%. 

Crude oil prices also moved higher on hopes a production freeze agreement among the Organization of Petroleum Exporting Countries would begin to correct a years-long decline in commodities. Saudi Oil Minister Khalid al-Falih reiterated his view that markets would begin to rebalance and stabilize in the first half of this year in response to the agreement that went into effect at the beginning of January.

West Texas Intermediate crude oil was up 1.1% to $52.96 a barrel on Tuesday morning.

Business conditions in the New York region weakened in January, according to the latest Empire State Manufacturing Survey. The index fell to 6.5 in January, down from 7.6 in December. Conditions were expected to improve this month. 

Morgan Stanley (MS - Get Report) rose 0.8% after reporting an 83% surge in quarterly earnings thanks to a year-end boost to trading revenue. Quarterly profit increased to 81 cents a share, up from 39 cents in the same quarter a year earlier and above consensus of 65 cents a share. Sales increased 17% to $9.02 billion, above estimates of $8.47 a barrel.  

UnitedHealth (UNH - Get Report) reported adjusted earnings of $2.11 a share in the fourth quarter, topping estimates of $2.07. Quarterly profit rose 56% over the quarter as premiums continued to rise. The largest U.S. health insurer also reported a 9% increase in revenue, above estimates, while backing its forecasts for full-year earnings growth. The stock rose 0.6% in premarket trading.

British American Tobacco (BTI)  said Tuesday it reached an agreement to buy the remaining stake of Reynolds American (RAI)  it doesn't already own for just under $50 billion. The cash-and-share deal pegs the deal's value at around $49.4 billion. Reynolds American rose 4% in premarket trading.

Syngenta (SYT) increased 2% on hopes its $43 billion takeover of ChemChina would soon pass regulatory muster. The agricultural chemicals company said it is "working well with the U.S. and the EU regulators now toward finalizing the agreements." CEO Erik Fyrwald said he anticipates a deal in the "not too distant future." 

Noble Energy (NBL - Get Report) will buy Clayton Williams Energy (CWEI) for about $2.7 billion in a cash-and-stock deal. The deal expands Noble Energy's holdings in the sought-after Permian Basin. The merger is expected to close in the second quarter of 2017.

American Express (AXP - Get Report) climbed nearly 1% after being upgraded to overweight from neutral at JPMorgan. Meanwhile, JPMorgan also downgraded Chipotle (CMG - Get Report) to neutral from overweight, sending shares sliding nearly 1%.

Nordstrom (JWN - Get Report) fell after being downgraded to hold from buy at Stifel with its price target slashed to $40 from $62. Analysts said they anticipate holiday sales will be less than expected, noting that the "highly competitive environment for retail" has affected profit margins and sales.