SEOUL - A spokesman for the defence ministry in Seoul said the test appeared to be of two "multiple-launch rockets" with a range of around 180 kilometres (110 miles).
It marks the third such test by the North in the past week - all three involving firing into the Sea of Japan (East Sea).
Japan, which is currently engaged in talks with North Korea on the abduction of Japanese citizens during the Cold War, has officially protested the tests, calling them "extremely regrettable".
The latest launches came on the eve of Mr Xi's arrival in Seoul on Thursday for a summit with South Korean President Park Geun Hye, during which the issue of North Korea's nuclear weapons programme will figure high on the agenda.
As North Korea's chief ally and economic benefactor, China is widely seen as the member of the international community with the most leverage over the regime in Pyongyang.
Seoul and Washington have persistently called on Beijing to exert more pressure on the North to abandon its nuclear ambitions.
It will be Ms Park's second summit with Mr Xi, who has notably not yet met with North Korean leader Kim Jong Un since the latter came to power following the death of his father, Kim Jong Il, in December 2011.
The test firings over the past week have been seen as a bout of military muscle flexing by the North, showing the range of its missile arsenal.
The first test on Thursday last week was hailed by the state media as that of a new "cutting-edge" guided missile which marked a "breakthrough" in the North's military capabilities.
The South Korean military said the second test on Sunday was of two short-range Scud missiles with a range of about 500 kilometres.
The South's Unification Ministry said there were a number of possible motives for the multiple tests, ranging from a display for domestic consumption, to a show of strength for the international community or a warning to Seoul.
"Whatever the reason, it's our position that any moves that can escalate military tensions in the Korean peninsula should be stopped," said ministry spokesman Kim Eui Do.
North Korea has been blowing hot and cold in recent weeks, one day threatening a "devastating strike" against the South and the next offering an olive branch to its perennial rival.
On Monday, the North's top military body, the National Defence Commission (NDC), proposed that the two Koreas suspend hostile military activities, along with all acts of verbal provocation and slander. South Korea on Tuesday rejected the offer as "nonsensical", and suggested that Pyongyang show its sincerity by dumping its nuclear weapons.
Although the 1950-53 Korean conflict ended six decades ago, the fact that it concluded with a ceasefire rather than a peace treaty means the two Korea remain technically at war.