North Korea launched ballistic missiles on Sunday, the day the leader of rival South Korea starts a state visit to China, Pyongyang’s chief ally, and just hours after the US attacked Venezuela.

The firings of at least two missiles, the country’s first in two months, further heighten global tensions after US President Donald Trump launched an assault in Venezuela in which President Nicolas Maduro was captured.

North Korea strongly denounced the US action, saying Washington "wildly violated the sovereignty of Venezuela" and the act shows "the rogue and brutal nature of the US."

The North launched its missiles hours before South Korean President Lee Jae Myung began a state visit to China on Sunday in the hope of promoting peace on the Korean Peninsula during a summit with his counterpart Xi Jinping.

The launches from the capital Pyongyang into the sea between the Koreas and Japan represent "a message to China to deter closer ties with South Korea and to counter China's stance on denuclearisation", said Lim Eul-chul, a professor at the Institute for Far Eastern Studies in Seoul.

North Korea signals 'We are different from Venezuela'

He said North Korea also wanted to send a message that "we are different from Venezuela" - as a nuclear and military power, ready to respond with "aggressive deterrence".

Referring to North Korea's leader Kim Jong Un, Bong Youngshik, a visiting professor at Yonsei University, said: "After seeing what’s happening in Venezuela right now, the person who would be most afraid is Kim Jong Un."

Seoul and Tokyo criticised the North's missile launches.

South Korea's presidential office said it had held an emergency security meeting and urged North Korea to cease "provocative acts that violate United Nations Security Council resolutions".

Japanese Defense Minister Shinjiro Koizumi said the launches threaten the peace and security of the region and the international community.

"Our government lodged a strong protest with North Korea and strongly condemned it," Koizumi said in a statement.

US forces for the Indo-Pacific said in a statement, "This event does not pose an immediate threat to US personnel or territory, or to our allies," adding the US was consulting closely with its allies and partners.

Kim Jong Un flexing military muscles before party congress

The missiles, launched around 7:50 am(2250 GMT on Saturday), flew about 900 km (560 miles), South Korea's military said. Japan said there were at least two missiles that flew about 900 km and 950 km.

The last time Pyongyang tested a ballistic missile was on November 7.

On Saturday, Kim Jong Un called for more than doubling the production capacity of tactical guided weapons during a visit to a munitions factory, North Korea's state media reported.

In recent weeks, Kim has made a series of visits to weapons factories, as well as to a nuclear-powered submarine, and has overseen missile tests ahead of this year's Ninth Party Congress of the Workers' Party, which will set out major policy goals.

South Korea expects Beijing to play a role in promoting peace on the Korean peninsula, said Wi Sung-lac, Lee's security adviser, without elaborating on details of the summit agenda.

Lee's agenda with Xi includes persuading China to facilitate dialogue with North Korea, experts say, at a time when North Korea has dismissed an outreach from Lee, who took office seven months ago.