SEOUL, South Korea -- Tensions mounted over North Korea's threat to test its first atomic bomb, with shots ringing out Saturday along the border with South Korea and Japan warning of harsh sanctions if Pyongyang goes nuclear.

With a possible test expected as early as Sunday, the U.N. Security Council issued a stern statement Friday urging the country to abandon its nuclear ambitions and warning of unspecified consequences if the isolated, communist regime doesn't comply.

Jittery nations have warned a test would unravel regional security and possibly trigger an arms race.

A midday incursion Saturday by North Korean troops into the southern side of the no-man's-land separating North and South Korea only stoked the unease.

South Korean soldiers rattled off 40 warning shots at the five communist troops who crossed the center line of the Demilitarized Zone, the inter-Korean buffer.

It was unclear whether the North Korean advance was intended as a provocation, or was an attempt to go fishing at a nearby stream, an official at South Korea's Joint Chiefs of Staff said on condition of anonymity, citing official policy. No one was hurt, and the North Koreans retreated.

While such border skirmishes are not unheard of, they are relatively rare. Saturday's incursion was only the second this year, the official said.

Meanwhile, world powers were stepping up diplomatic efforts to avert a nuclear test. Japanese Prime Minister Shinzo Abe was to visit Beijing on Sunday for talks with Chinese President Hu Jintao and then proceed to Seoul for talks with South Korean President Roh Moo-hyun the following day.

A State Department spokesman, Kurtis Cooper, said Saturday the United States was concerned about North Korea's threat to test its first atomic bomb and that the department was closely monitoring the high tensions.

Also Saturday, South Korea's nuclear envoy announced he will visit Beijing on Monday for two days of talks with Chinese officials about the threatened nuclear test.

In a separate statement from Tokyo, Japan's Foreign Ministry said it was prepared to push for punitive measures at the United Nations if the North goes ahead with the test.

"If North Korea conducts a nuclear weapons test despite the concerns expressed by international society, the Security Council must adopt a resolution outlining severely punitive measures," the ministry said.

Japan plans to step up economic sanctions against North Korea, tighten trade restrictions and freeze additional North Korea-linked bank accounts should a nuclear test be carried out, Japan's Nihon Keizai newspaper reported.

The U.N. statement adopted Friday expressed "deep concern" over North Korea's announcement Tuesday that it is planning a test.

The council acted amid speculation that a nuclear test could come on Sunday, the anniversary of North Korean leader Kim Jong Il's appointment as head of the Korean Workers' Party in 1997.

Japanese Vice Foreign Minister Shotaro Yachi told Japan's TV Asahi: "Based on the development so far, it would be best to view that a test is possible this weekend."

The U.N. statement also urged North Korea to return to six-nation negotiations aimed at persuading the country to abandon its nuclear ambitions in exchange for security guarantees and badly needed economic aid.

Those talks, which involve the United States, China, Japan, Russia and North and South Korea, have been stalled since late last year, when North Korea boycotted the negotiations in response to American economic sanctions.

A North Korea expert in China, the North's closest ally, said only the removal of the sanctions could dissuade the North.

"North Korea has already made a decision to carry out a test," said Li Dunqiu of China's State Council Development Research Center, a Cabinet-level think tank. But "if the U.S. removes sanctions ... then tensions can be eased. Otherwise launching a nuclear test is unavoidable for North Korea."

The United States imposed economic restrictions on North Korea last year to punish it for alleged counterfeiting and money laundering.

North Korea said Tuesday it decided to act in the face of what it claimed was "the U.S. extreme threat of a nuclear war," but gave no date for the test. Washington has repeatedly said it has no intention of invading North Korea.