The broader Nifty fell below the 9,850 level to 9,837.85.
Barring realty and metal sectors, which witnessed value buying, investors opted for profit-booking ahead of the September derivative expiry on Thursday.
The Sensex traded at 31,517.99 at 1203 hours, down 108.64 points, or 0.34 per cent.
The broader Nifty declined 34.75 points, or 0.35 per cent, to 9,837.85 at 1203 hours.
North Korea's foreign minister said US President Donald Trump had declared war on the country and it reserves the right to take countermeasures, including shooting down US bombers even if they are not in its airspace.
Foreign portfolio investors (FPIs) sold shares worth Rs 1,249.45 crore yesterday, showed provisional data from the stock exchanges. Domestic institutional investors (DIIs) bought shares worth a net Rs 1,009.98 crore.
Overseas, most Asian stocks dropped taking cues from weak close in US stocks overnight, hurt by fresh threats from North Korea and weakness among key technology stocks.