Aeorema Comm. Share News (AEO)

* Trump to make announcement on tariffs later on Thursday

* Express Scripts jumps after Cigna's $53 bln bid

* Dow down 0.28 pct, S&P down 0.09 pct, Nasdaq down 0.02 pct(Updates to late, changes byline)

By Sinéad Carew

March 8 (Reuters) - The S&P 500 was roughly flat on Thursdayand oscillated between positive and negative territory asinvestors were nervous of making bets ahead of an announcementfrom President Donald Trump on metal tariffs after a volatileweek marked by worries about a potential trade war.

Trump is expected to formalize 25 percent import duties onsteel and a 10 percent tariff on aluminum at 3:30 p.m ET (2030GMT), after saying Canada and Mexico would be exempted if theyagree on a trade deal with the United States.

But Trump told reporters ahead of the announcement that hehad a right to exempt or add countries, adding to uncertaintyamong market participants.

Worries that the tariffs would ignite a global trade warhave dominated markets since Trump announced to tariff plan lastThursday, and the exit of chief economic adviser Gary Cohn lateTuesday intensified such concerns.

"We've been busy buying and selling the rumor. We want toknow what the news is. Announcing at the beginning of thetrading day this would happen at the end was not great formarket action. It ramped up uncertainty," said Kim Forrest,senior portfolio manager at Fort Pitt Capital in Pittsburgh.

"He has a pattern of coming out hard and walking it back.Now we'll see what he means."

At 2:10 p.m. ET, the Dow Jones Industrial Averagefell 69.67 points, or 0.28 percent, to 24,731.69, the S&P 500lost 2.32 points, or 0.09 percent, to 2,724.48 and theNasdaq Composite dropped 1.45 points, or 0.02 percent,to 7,395.20.

Major trading partners Europe and China have warned theywould respond to any action by the United States.

"If he (Trump) does announce much more draconian measuresthat are going to knock on to other products, I think there willbe a sell-off," said Dec Mullarkey, managing director at SunLife Investment Management based in Wellesley, Massachusetts.

Express Scripts was among the top boosts for theS&P 500, rising 8 percent after health insurer Cignaagreed to buy the pharmacy benefits manager for almost $53billion. Cigna shares were the biggest drag with a 11.2 percentdrop.

Kroger fell more than 12 percent after thesupermarket chain's disappointing full-year profit forecast.

American Eagle Outfitters fell 9.5 percent onconcerns over falling margins.

Declining issues outnumbered advancing ones on the NYSE by a1.23-to-1 ratio; on Nasdaq, a 1.23-to-1 ratio favored decliners.

The S&P 500 posted 22 new 52-week highs and 1 new low; theNasdaq Composite recorded 145 new highs and 19 new lows.(Reporting by Sruthi Shankar in Bengaluru; Editing by SrirajKalluvila and Chizu Nomiyama)