South Korea said the fishing boat sailed about 200 yards into its waters and the South's navy fired eight machine-gun rounds. There were no reports of injuries.
The Southern navy boat was about 300 yards from the fishing boat and no hostile actions were taken by North Korean navy ships in the area, said Kim Sung-ok, a spokesman at the South Korean military's Office of Joint Chiefs of Staff.
The North Korean fishing boat turned back in five minutes, Kim said.
South Korea's navy ships had also fired warning shots Sunday when eight North Korean fishing boats allegedly entered the South's territorial waters.
Tension along the maritime border comes as the United States is mustering international pressure on the North to abandon its suspected development of nuclear weapons.
"Now that there is an increasing danger of a nuclear war on the Korean peninsula, any military clash between the North and the South may lead to a war," a spokesman for the North Korean Navy Command told Pyongyang's official news agency KCNA.
The unidentified spokesman accused South Korea of deploying warships closer to the border off the peninsula's west coast and infiltrating warships into the North's territorial waters "every day, escalating the tensions."
South Korea is "staging a prelude" to a sea skirmish to "provide the U.S. imperialist warhawks, who are blustering that North Korea is next to Iraq, obsessed with war hysteria, with favorable conditions for a war," he said.
South Korea denies such accusations.
This year, the North has conducted missile tests and intercepted a U.S. surveillance plane, maneuvers that were viewed as attempts to pressure the United States into negotiations.
U.S. officials say that at talks in Beijing in April, North Korea claimed that it had nuclear weapons but was willing to scrap its nuclear programs in return for security guarantees and economic aid.
This week, U.S. lawmakers say North Korean officials told them that they had nuclear weapons and were nearly done reprocessing about 8,000 spent fuel rods, which would yield material for even more weapons.
Mr. Bush says he prefers a diplomatic end to the standoff, but has not ruled out a military option.
On Monday, Group of Eight world leaders meeting in France accused North Korea of undermining non-proliferation agreements.
"North Korea's uranium enrichment and plutonium production programs and its failure to comply with its IAEA safeguards agreement undermine the non-proliferation regime and are a clear breach of North Korea's international obligations," the statement read. "We strongly urge North Korea to visibly, verifiably and irreversibly dismantle any nuclear weapons programs, a fundamental step to facilitate a comprehensive and peaceful solution."
A senior U.S. official, speaking on condition of anonymity, said the United States reads the G-8 declaration as implicitly authorizing the use of force against countries that violate international nonproliferation norms.
But French President Jacques Chirac bluntly said, "This interpretation seems to be extraordinarily daring."
"There never was any talk of using force whatsoever. We have to have a dialogue with Iran," Chirac said at his closing news conference Tuesday.
The administration is taking steps toward upgrading defenses in the region. The Pentagon's No. 2 official said Tuesday that U.S. forces must modernize to better counter a potential attack by North Korea, adding that Washington would send its chief missile commander to consult with Japan about missile shields.
Deputy Defense Secretary Paul Wolfowitz said the changes were part of a review of deployment of U.S. forces worldwide. But he hinted it could mean smaller, more mobile forces for a region overshadowed by North Korean nuclear weapons.
Talk of American troop redeployments has been a touchy subject for between Washington and its regional allies, Japan and South Korea.
Defense Secretary Donald Rumsfeld spurred concern in April by saying troops stationed near the demilitarized zone between the two Koreas could be shifted south, moved to other countries in the region or even brought home.
At the same time, media reports have said the Pentagon is considering the withdrawal of three-fourths of the approximately 20,000 U.S. Marines stationed on the southern Japanese island of Okinawa.