At 9:39 a.m. ET (1339 GMT), the Dow Jones Industrial Average was up 33.52 points, or 0.15 percent, at 22,301.86, the S&P 500 was up 4.44 points, or 0.17 percent, at 2,504.67 and the Nasdaq Composite was up 18.27 points, or 0.28 percent, at 6,466.74.
At a two-day meeting beginning later on Tuesday, the Fed is expected to take another step toward policy normalization and announce plans to begin unwinding its $4.2 trillion portfolio of Treasuries and mortgage-backed securities.
President Donald Trump gave his first address to the United Nations General Assembly, and at one point stated that the US was ready, willing and able to act against North Korea militarily, and that the USA would totally destroy the country if necessary.
Expectations that the Fed may raise rates in December have increased since data last Thursday showed that US consumer prices accelerated in August.
In the bond market, treasuries have moved slightly higher ahead of the Fed announcement. It will issue new economic forecasts and Fed Chair Janet Yellen holds a news conference.Markets are pricing in a 56 percent probability of the Fed raising rates in December, according to the CME Group's FedWatch tool.US 10-year Treasury yields, which edged up on Tuesday, retreated 1.8 basis points on the day to 2.23 percent.
US stocks are mixed early Wednesday, treading water ahead of the Federal Reserve's policy directive, due out this afternoon.
The Standard & Poor's 500 index inched up 1.59 points, or 0.1 percent, to 2,508.24. The pound, which had surged last week when the Bank of England hinted a rate increase was coming, was at $1.3544, from $1.3592.
The prospect of another Fed rate hike this year at a time when the US economy is growing modestly and may slow somewhat from the impact of hurricanes Harvey and Irma, could be bad news for stocks the next few weeks, Chalupnik said. "And it's at risk because of lower economic numbers, higher interest rates and earnings that, on an individual-company basis, could disappoint if they were impacted by hurricanes Harvey and Irma".
However, the dollar maintained its recent strength against the yen, holding well above 111 yen. CalAtlantic Group fell the most, shedding 97 cents, or 2.7 percent, to $34.60.
U.S. Defense Secretary Jim Mattis hinted on Monday about the existence of military options on North Korea that might spare Seoul from a brutal counterattack but declined to say what kind of options he was talking about or whether they involved the use of lethal force.
Advancing issues outnumbered decliners on the NYSE by 1,705 to 773.
Among metals, gold gained $5.80 to $1,316.40 an ounce. On the Nasdaq, 1,137 issues fell and 1,089 advanced. While the U.K.'s FTSE 100 Index is down by 0.2 percent, the German DAX Index and the French CAC 40 are both almost flat. In Asia, Japan's Nikkei 225 added 0.1 percent and South Korea's Kospi slipped 0.2 percent.