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Trump also put more economic pressure on North Korea by ordering additional sanctions, including on its shipping and trade networks but stopped short of going after Pyongyang's biggest trading partner, China.
"Big noise out of North Korea will keep today's trading defensive as the biggest threat to the markets make the headlines," said Peter Cardillo, chief market economist at First Standard Financial in New York.
"While we don't expect a serious selloff, the geopolitical rhetoric could heat up."
The latest spike in tensions prompted demand for gold, which rebounded from a four-week low on Friday.
Investors increased bets the Federal Reserve would raise rates again this year after its economic projections showed 11 of 16 officials see room for a 0.25 percentage point increase in the federal funds' rate from the current level.
Market will look for more insights into Fed's policy from speeches by Dallas Fed president Robert Kaplan and his Kansas City counterpart Esther George later in the day.
Oil prices were steady as investors waited to see whether major producers meeting in Vienna would back an extension to output cuts beyond March next year.
At 9:38 a.m. ET (1338 GMT), the Dow Jones Industrial Average was down 18.53 points, or 0.08 percent, at 22,340.7, the S&P was down 2.31 points, or 0.09 percent, at 2,498.29 and the Nasdaq Composite was down 7.28 points, or 0.11 percent, at 6,415.41.
Six of the 11 major S&P sectors were higher, led by 0.56 percent rise in the telecom services index. The financial index fell 0.44 percent, its first session of loss in six days.
T-Mobile rose 1.45 percent after Reuters exclusively reported the company was close to agreeing on tentative terms on a deal to merge with Sprint. Sprint shares jumped 4.92 percent.
Versartis slumped more than 85 percent after its experimental drug for treatment of growth hormone deficiency in children failed in a late-stage study.
United States Steel dipped 5.19 percent after Cowen downgraded the stock to "underperform" from "market perform".
Advancing issues outnumbered decliners on the NYSE by 1,435 to 997. On the Nasdaq, 1,108 issues fell and 1,064 advanced.