The two-month-long "Foal Eagle" air, ground and naval field training exercise involved more than 10,000 US troops along with a far higher number of South Korean personnel.
"The drill is over but the South Korean and US militaries will continue to watch out for potential provocations by the North, including a missile launch," Seoul's defence ministry spokesman Kim Min-Seok told reporters.
The North is still maintaining a number of missiles and missile launchers that were recently moved to its east coast in apparent preparation for a launch, Kim added.
"With the military drills over, at least we can worry less about any accidental clash developing into a full-scale war," said Paik Hak-Soon, a North Korea expert at the Sejong Institute, a think-tank in Seoul.
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He said a planned summit in Washington on May 7 between the US President Barack Obama and South Korean leader Park Geun-Hye -- who took office in February -- could be more significant in setting the tone for inter-Korean relations.
"If the North finds the outcome of the summit unsatisfying or unacceptable, that means we would have to live in constant fear of another military provocation near the border," Paik said.
Incensed by fresh UN sanctions and the joint South Korea-US military exercises, the North has spent weeks issuing blistering threats of missile strikes and war.
The Foal Eagle exercises "are the main factor of pushing the situation on the Korean Peninsula to the brink of a nuclear war", the newspaper of the North's communist party, the Rodong Sinmun, said on Monday.
"The U.S. And South Korean warmongers should bear in mind that they will not be able to escape a miserable doom if they ignite a nuclear war against the DPRK in the end," it added.
Pyongyang has regularly accused the US of preparing to launch a nuclear strike on its territory, and reacted furiously to the use of nuclear-capable B-52s and B-2 stealth bombers in the joint South-US drills.