The session-over-session changes in the dollar-won exchange rates averaged 0.42 percent in July, compared with 0.28 percent a month earlier, the Bank of Korea said in a statement.
"The won has fallen slightly against the dollar since July due to geopolitical risks" stemming from North Korea's missile and nuclear programs, the statement said.
North Korea and the United States have traded bellicose rhetoric after the United Nations Security Council further tightened sanctions on North Korea for its two tests last month of intercontinental ballistic missiles.
On Wednesday, North Korea's military said it is considering missile strikes near the US territory of Guam, warning that it is ready to stage all-out war if Washington launches a preventive war against it.
The North's threat came hours after US President Donald Trump warned that any threat by North Korea to the US will be met with "fire and fury."
Meanwhile, the Korean won posted 5.2 won in daily value changes against the US dollar in July, up from 4.5 won a month earlier, it said.
The won finished at 1,125.0 against the greenback on Aug. 4, compared with 1,119.0 won at the end of July, the BOK said.
Foreign investors poured a net $2.55 billion into the local financial markets -- $110 million in stocks and $2.45 billion in bonds -- in July, the BOK said. It was down from foreign injections worth $3.09 billion a month earlier.
The credit default swaps premium for foreign exchange stabilization bonds came to 58 basis points in July, up from 52 basis points a month earlier, the BOK said. (Yonhap)