Tensions are on high along the border between North and South Korea after southern troops fired into the inter-Korean Demilitarized Zone Saturday -- on a weekend when the world is waiting to see if the North will conduct a nuclear test.

North Korea could carry out testing of its first atomic bomb as early as Sunday despite a stern UN Security Council warning on Friday.

"The Security Council stresses that a nuclear test . . . would represent a clear threat to international peace and security and that should the DPRK (Democratic People's Republic of Korea) ignore calls of the international community, the Security Council will act consistent with its responsibility under the Charter of the United Nations," the statement said.

On Saturday, South Korean soldiers fired 40 warning shots at a group of five North Korean soldiers who crossed the centre line of the shared Demilitarized Zone.

It is unclear if North Korea sent the group as an intentional provocation or if they were going fishing at a nearby stream, an anonymous South Korean official from the Joint Chiefs of Staff told The Associated Press.

Only one of the five men was armed, said officials.

Similar border scuffles are relatively rare, with Saturday's incident only the second this year.  The buffer zone was created with the declaration a ceasefire in the Korean War more than 50 years ago (the two sides are technically still at war).

Diplomatic struggle

A host of diplomatic meetings throughout Asia are to be held in the coming days in an effort to prevent a nuclear test.

Japanese Prime Minister Shinzo Abe will visit China's President Hu Jintao, whose country is viewed as the North's closest ally, on Sunday. South Korean officials will follow on Monday.

Abe will meet with South Korean President Roh Moo-hyun on Monday.

Japan plans to impose harsher economic sanctions and trade restrictions if North Korea goes ahead with the test.

"Based on the development so far, it would be best to view that a test is possible this weekend," Japanese Vice-Foreign Minister Shotaro Yachi told a local television station.

Sunday has been earmarked as a possible test date since it marks the anniversary of North Korean leader Kim Jong Il's 1997 appointment as head of the Korean Workers' party.

North Korea boycotted six-country negotiations last year after the U.S. imposed economic sanctions on the country over counterfeiting and money laundering allegations.

The talks, involving the U.S., China, Japan, Russia and South Korea were aimed at ending North Korea's nuclear ambitions in exchange for economic aid and security guarantees.

Li Dunqiu, of China's State Council Development Research Centre (a cabinet-level think tank), said North Korea has already decided to conduct the test.  Dunqiu said the only thing that could possibly prevent a test is if the U.S. removes the sanctions.

"If the U.S. removes sanctions ... then tensions can be eased.  Otherwise launching a nuclear test is unavoidable for North Korea," said Li.

North Korea justified their actions on Tuesday saying they were acting in defiance of "the U.S. extreme threat of a nuclear war."

With files from The Associated Press