Tensions on the Korean peninsula have been running sky-high for weeks, with signs that the North might be preparing a long-range missile launch or a sixth nuclear test -- and with Washington refusing to rule out a military strike in response.
The massive "Foal Eagle" drill, which the defence ministry in Seoul said was ending as scheduled on Sunday, involved around 20,000 South Korean and 10,000 US troops.
Another annual joint exercise known as "Key Resolve" ended last month.
Both play out scenarios for a conflict with North Korea, but Seoul and Washington insist they are purely defensive in nature, despite Pyongyang's claims that they are provocative rehearsals for invasion.
Their conclusion normally signals a period of relative calm in North-South tensions, but this year the situation looks set to remain highly volatile.
US President Donald Trump has warned of a possible "major conflict" while Pyongyang has carried out a series of failed missile tests, including one on Saturday, and a massive live-fire military exercise.
The South Korean defence ministry confirmed Sunday that a joint naval drill with a US strike group, led by the aircraft carrier USS Carl Vinson, was still ongoing in the Sea of Japan (East Sea).
The exercise, aimed at verifying the allies' capability to track and intercept enemy ballistic missiles, is expected to continue until sometime next week.
Through state media, North Korea has threatened to attack the Carl Vinson, and a state-sponsored website on Sunday also warned of a possible strike against a US nuclear-powered submarine despatched to the area.
China is "putting pressure" on its ally North Korea to curb its weapons programmes, Trump told the CBS television network's "Face the Nation" programme.
If North Korea carries out another nuclear test "I would not be happy," he said.
"And I can tell you also, I don't believe that the president of China, who is a very respected man, will be happy either," Trump said in excerpts of the interview released Saturday.
Asked if "not happy" signified "military action," Trump answered: "I don't know. I mean, we'll see."
'Catastrophic consequences'
Pyongyang's show of defiance included a failed missile test on Saturday that came just hours after US Secretary of State Rex Tillerson warned the UN Security Council of "catastrophic consequences" if the international community -- most notably China -- failed to pressure the North into abandoning its weapons programme.
Military options for dealing with the North were still "on the table", Tillerson said.
China has repeatedly pushed back at the idea that it alone holds the solution to curbing the North's nuclear ambitions, and warned that any use of US force would only lead to "bigger disasters".
Pope Francis this weekend called for negotiations to resolve tensions over North Korea.
"There are plenty of mediators in the world who are putting themselves forward. Norway, for example which is ready to help," he said.
The tensions have also triggered some friction between Seoul and Washington, with Trump suggesting in a recent interview that the South should pay for the $1.0-billion dollar THAAD anti-missile system that the US is deploying on its ally's territory.
But on Sunday South Korea said US National Security Adviser H.R. McMaster had spoken by phone with his counterpart in Seoul and both sides had reaffirmed that Washington would bear the cost of the THAAD deployment, as initially agreed.
Trump's interview remarks "were made in a general context, reflecting the American public's hopes for (defense) cost sharing," McMaster was quoted as saying by the South's presidential office.
The two countries have had a security alliance since the 1950-53 Korean war, and more than 28,000 US troops are stationed in the South.
Beijing has been infuriated by the THAAD deployment, which it says upsets the regional security balance.
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