The United States and Japan have vowed to work closely with the new South Korean government, while China said it hopes political stability will be restored quickly, following President Park Geun Hye's ouster over a corruption scandal.
US State Department spokesman Mark Toner called South Korea a "steadfast ally" and said the US will forge a "productive relationship" with its next president, who has to be elected by May 9.
Japanese Foreign Minister Fumio Kishida said the East Asian neighbours share strategic interests, adding: "The cooperation and coordination between Japan and South Korea are crucial to regional peace and stability."
Of deep concern to the US-Japan- South Korea defence triangle is the threat of a belligerent Pyongyang, which on Monday fired four ballistic missiles in what it said was a dry run for a strike on US bases in Japan.
To guard against the North, Ms Park had given the go-ahead for the US Terminal High Altitude Area Defence (Thaad) anti-missile system to be deployed by this year.
But Beijing views Thaad as a security threat and has been taking economic retaliatory measures against Seoul.
"During her term of office, Ms Park did a lot to promote Sino- South Korean ties. But her government's decision to deploy Thaad has also affected relations," Chinese Foreign Ministry spokesman Geng Shuang said yesterday.
"We hope South Korea can use our bilateral ties as the starting point to remove all obstacles."
Tokyo-Seoul ties are also strained over unresolved comfort women issues. Japan recalled its ambassador to South Korea Yasumasa Nagamine in January, but there is still no sign of when he will return.
Mr Kishida, in urging Seoul to uphold its end of the bargain of a landmark comfort women deal struck in 2015, said Tokyo will decide on its envoy's return after "considering various aspects comprehensively".
At the same time, he stressed Japan's need to "promote cooperation with the new South Korean government in various areas".
The future of ties, however, is uncertain. Unlike Ms Park, the presidential front runner, Mr Moon Jae In, opposes the comfort women deal as well as an intelligence-sharing pact that Seoul has inked with Tokyo. Mr Moon, the former head of the opposition Democratic Party, also opposes Thaad, preferring closer engagement with the North.