US STOCKS-Wall St falls, Nasdaq under 50-day moving average

* Ukraine conflict continues; Russia seizes marine base

* Weak China data raises hopes for stimulus

* Nasdaq hit as Internet, biotech names tumble

* Herbalife (NYSE: HLF - news) rallies, Icahn to get more reps on board

* Dow off 0.4 pct; S&P 500 down 0.7 pct; Nasdaq off 1.7 pct (Updates to early afternoon)

By Ryan Vlastelica

NEW YORK (Frankfurt: HX6.F - news) , March 24 (Reuters) - U.S. stocks fell sharply on Monday, with the Nasdaq marking its biggest daily percentage drop since early February as some of the market's recent best performers like technology and biotech shares led the way down.

Concerns that the crisis in Ukraine could escalate pushed traders to drop some of the market's biggest trading favorites, taking the Nasdaq below its 50-day moving average in a sign of weakening near-term momentum.

Almost 80 percent of the stocks traded on the Nasdaq were lower, while about two-thirds of New York Stock Exchange-listed shares fell. Eight of the 10 S&P 500 sectors slid for the day.

Netflix Inc (NasdaqGS: NFLX - news) was the S&P 500's biggest percentage loser, down 8.9 percent at $369.72, with fellow Internet names Facebook Inc (NasdaqGS: FB - news) and TripAdvisor Inc (NasdaqGS: TRIP - news) close behind, both falling 5 percent. All three names had scored sharp gains in 2013.

The Nasdaq biotechnology index, which jumped 66 percent last year, fell 3.5 percent in its fourth straight daily decline, a period over which the group has lost 9 percent. Among specific names, Alexion Pharmaceuticals Inc (NasdaqGS: ALXN - news) dropped 6.1 percent to $150.12.

"Biotech stocks have gone parabolic over the past few months, so this is a necessary correction to that," said Mark D'Cruz, senior investment analyst at Key Private Bank in Cleveland, Ohio. "A lot of that interest came from outside traditional biotech investors, who are now being scared off ... Biotechs really have to prove themselves this year, prove that their drugs can deliver."

Ukraine announced the evacuation of its troops from Crimea, essentially yielding the region to Russian forces, which seized a Ukrainian marine base. While few U.S. companies have excessive exposure to the region, investors are concerned about the potential economic fallout from any escalation in tensions.

U.S. President Barack Obama, who has imposed personal sanctions against some of Russian President Vladimir Putin's political and business allies, began crisis talks with his European allies over how to respond in the biggest East-West conflict since the Cold War.

"The issue remains contained for the time being, but Obama will try and garner support for more sanctions, which will ultimately shape our view of how things can end up looking," said Art Hogan, chief market strategist at Wunderlich Securities in New York. "This remains at the forefront of what we're paying attention to."

The Dow Jones industrial average was down 63.36 points, or 0.39 percent, at 16,239.41. The Standard & Poor's 500 Index was down 13.62 points, or 0.73 percent, at 1,852.90. The Nasdaq Composite Index was down 70.40 points, or 1.65 percent, at 4,206.39.

Futures had been higher throughout the premarket session as hopes grew that China would take new stimulative measures to support its economy after data showed that the nation's manufacturing contracted in the first quarter of 2014.

The Dow's losses were limited by a rise in Procter & Gamble Co, a consumer staple that is considered a defensive play. That stock rose 2.2 percent to $79.57, snapping a four-day losing streak.

In the latest snapshot of the U.S. economy, financial data firm Markit said its preliminary read on March manufacturing activity slowed after nearing a four-year high last month. Markit, however, said the rate of growth and the pace of hiring remained strong.

Herbalife Ltd said it would allow three more representatives of billionaire investor Carl Icahn, the company's biggest shareholder, to join its board. Shares of the nutrition and weight-loss company jumped 7.2 percent to $53.10.

NU Skin Enterprises Inc (NYSE: NUS - news) soared 18.6 percent to $88.93 on heavy volume. Earlier, China fined the company more than $500,000 for illegal product sales and misleading local consumers, China's State Administration for Industry & Commerce said in a statement on its website. NU Skin said last week that it expected a fine. (Editing by Bernadette Baum, Nick Zieminski and Jan Paschal)