"North Korea fired off 30 short-range missiles between 4am and 6.10am (1900-2110 GMT Friday) this morning from its East coast into the Sea of Japan (East Sea)," said a spokesman for the Joint Chiefs.
"The missiles are estimated to have flown about 60km (37 miles)," he added.
Analysts said the missiles had been launched from the same location as 25 projectiles on Sunday, near the eastern port of Wonsan. The projectiles were Soviet-era short range Frog missiles from the 1960s, they said.
"This is an expression of anger at the joint military exercises" South Korea has been staging with its ally the United States, Prof Yang Moo-Jin of the University of North Korean Studies told AFP.
It is not unusual for Pyongyang to carry out such tests but there has been a spate of them in recent weeks. Saturday's launch was the sixth in just over a month.
South Korea urged North Korea earlier this week to stop what it called "provocative" and potentially dangerous tests.
"The North should stop actions that cause military tension and unnerve its neighbours," Seoul's defence ministry spokesman Kim Min-Seok told reporters on Monday.
"Provocative action made without any prior notifications... can pose significant danger to sea vessels and aircraft passing by the area," he added.
The US State Department had also called on Pyongyang to refrain from "provocative actions that aggravate tensions".
Beijing expressed concern earlier this month after the North test-fired a rocket into the flight path of a Chinese airliner.'Low-level provocation'
But Yang downplayed the danger.
"North Korea apparently decided to get rid of its rusting stockpile of some 100 Frog missiles by lobbing them into the sea as a show of force," he said. "This is merely a low-level provocation."
"The North is likely to test-fire all the remaining Frog missiles in the near future", he added.
The annual South Korean-US military drills started in late February and will run until mid-April.
The North has habitually criticised the Key Resolve and Foal Eagle exercises, along with other military drills south of the border, as rehearsals for an invasion.
Seoul and Washington say they are purely defensive.
Earlier this month, the North's powerful National Defence Commission threatened to demonstrate its nuclear deterrent in the face of what it called US hostility.
But Seoul's defence ministry said there was no sign of an imminent nuclear test by the North, which staged three atomic tests in 2006, 2009 and last year.
The latest missile tests came as South Korea and Japan said Friday that their leaders will hold a summit with US President Barack Obama next week, in a breakthrough after Washington urged the pair to mend badly strained ties.
The meeting on the sidelines of an international nuclear conference taking place in The Hague on Monday and Tuesday will mark the first formal talks between President Park Geun-Hye and Japan's Prime Minister Shinzo Abe since they took office more than a year ago.
"At the three-way summit, North Korea's nuclear programmes and the issue of nuclear non-proliferation will be discussed," Seoul's foreign ministry said in a statement.