Moody's downgraded Portugal's debt to junk status late Tuesday. That raised fresh concerns about the strength of the European financial system. The Euro Stoxx 50, an index of companies in countries that use the euro, fell nearly 1 percent.
China also raised a key interest rate for the third time this year in an attempt to curb inflation. Many U.S. companies have focused on the country as a source of profit growth and are hoping that interest rate hikes there will not lead to an economic slump.
The Dow Jones industrial average rose 7 points, or 0.1 percent, to 12,577 in early trading. The S&P 500 fell 4, or 0.3 percent, to 1,334. The Nasdaq composite dipped 4, or 0.2 percent, to 2,821.
Investors will receive a report later Wednesday on growth in the U.S. service sector. Economists expect the Institute for Supply Management's index expanded in June for the 19th straight month, although at a slower pace than in May. A surprisingly strong reading of the ISM's manufacturing index last week helped propel the Dow to its best weekly gains in two years.
Among U.S. companies, General Motors gained 2 percent after analysts upgraded the stock. Walgreen Co. rose nearly 1 percent after the retailer said its June sales were higher than anticipated.
There are no major corporate earnings reports scheduled for this week.
Many investors are now looking ahead to Friday's employment report. Economists anticipate that June's unemployment rate was unchanged from the month before at 9.1 percent.