North Korea views all such exercises as rehearsals for invasion and has repeatedly warned it would take "overwhelming" action in response.
North Korea fired a short-range ballistic missile, Seoul's military said, the fourth show of force in a week as South Korea and the United States stage major military drills.
"Our military detected one short-range ballistic missile fired from around the Tongchang-ri area in North Pyongan province at 11:05 am (0205 GMT) towards the East Sea," South Korea's Joint Chiefs of Staff said on Sunday, referring to the body of water also known as the Sea of Japan.
"Our military is maintaining a full readiness posture in close cooperation with the United States," it added.
Seoul and Washington have ramped up defence cooperation in the face of growing military and nuclear threats from the North, which has conducted a series of increasingly provocative banned weapons tests in recent months.
South Korea and the United States are currently in the middle of 11-day joint drills known as Freedom Shield, their largest in five years.
Japan's defence ministry also tweeted: "A possible ballistic missile was launched from North Korea."
Japan's coast guard warned vessels about "what appears to be a ballistic missile that was launched from North Korea" while noting that it seemed "to have already fallen".
READ MORE: ICBM launch serves as warning to US, South Korea over drills: DPRK
On Thursday, Pyongyang test-fired its largest and most powerful intercontinental ballistic missile, a Hwasong-17 -- its second ICBM test this year.
The North's state-run Korean Central News Agency (KCNA) described the ICBM launch as a response to the ongoing, "frantic" US-South Korea drills.
Analysts previously said North Korea would likely use the drills as an excuse to carry out more missile launches and perhaps even a nuclear test.
On Saturday, KCNA said the joint drills by the allies were "inching close to the unpardonable red-line".
The ICBM launch followed two short-range ballistic missiles on Tuesday, after two strategic cruise missiles were launched from a submarine last Sunday.
The recent flurry of aggression by Pyongyang has pushed Seoul and Tokyo to mend fences over historical disputes and try to boost security cooperation.
Just hours after the ICBM was fired Thursday, South Korean President Yoon Suk Yeol arrived in Japan for the first full-scale leaders' summit between the sides in 12 years.
Last year, North Korea declared itself an "irreversible" nuclear power, and leader Kim Jong Un recently called for an "exponential" increase in weapons production, including tactical nukes.
Kim earlier this month also ordered the North Korean military to intensify drills to prepare for a "real war".
READ MORE: Is the threat of North Korea spurring Japan-South Korea rapprochement?