U.S. stock index futures were little changed on Tuesday as fighting in Libya, resolving Japan's crisis and the chances for a European interest rate hike next month kept investors jittery.

Forces loyal to Muammar Gaddafi attacked two Libyan towns on Tuesday after a third night of air raids on Tripoli, but the Western military campaign faced questions over the future of its command structure.

European Central Bank President Jean-Claude Trichet and other ECB policymakers have reiterated they are ready to act quickly to guard against inflation.

In Japan, smoke and steam rose from two of the most threatening reactors at Japan's quake-crippled nuclear plant, suggesting the battle to avert a disastrous meltdown and stop the spread of radiation was far from won.

There is a lot of volatility and a lot of uncertainties surrounding Japan and Libya, Tim Ghriskey, chief investment officer at Solaris Asset Management in Bedford Hills, New York, said.

Neither one of the situations looks quite ready to settle down yet and that could drag out for some time and weigh on the market.

Furthermore, analysts expressed concern about the implications for inflation of rebuilding a post-earthquake Japan, the world's No. 3 economy.

The inflationary implications for the global economy of what comes next in Japan through massive imports of raw materials and central-bank money printing to finance it will be a main factor in how the world responds, said Peter Boockvar, Equity Strategist at Miller Tabak + Co in New York.

In company news, Sanofi-Aventis and Merck & Co abandoned plans for a joint animal health powerhouse with $5 billion in sales, citing the increasing complexity of disposals and regulatory reviews needed for the deal.

Design software maker Adobe Systems is expected to deliver its fourth consecutive quarterly improvement in profit, driven by strong adoption of its software. Wall Street has priced in a profit of 57 cents per share, up from 40 cents per share one year ago.

European shares pared gains as the latest developments in Libya promoted fresh jitters.

Wall Street advanced on Monday, with the Dow and the S&P rising 1.5 percent after AT&T said it would buy wireless rival T-Mobile USA, sparking expectations of more deal activity.

(Reporting by Angela Moon; Editing by Kenneth Barry)

Japan Libya Merck & Co. European union