Photo/IllutrationPeople sit in front of a TV screen showing a news program reporting about North Korea's missile firing, at Seoul Railway Station in Seoul on June 8. (AP Photo)

SEOUL--Activity has intensified in the area around a North Korean nuclear test site, indicating Pyongyang may be preparing a sixth nuclear test, which it warned last month was "imminent."

The preparations near the Punggyeri site match those of past occasions before North Korea conducted a nuclear test.

According to sources knowledgeable about what is occurring in North Korea, scientists who oversee nuclear materials as well as evaluate the nuclear tests have gathered at the Punggyeri site.

In addition, traffic to the site has been apparently shut down at the checkpoints leading to the area in northeastern North Korea.

However, movement of vehicles and humans within the test site continues to be active.

It is unclear if that flurry of activity is a precursor to a nuclear test or simply an exercise to prepare or inspect the site.

With the international community continuing to exert strong pressure on Pyongyang, a South Korean government source said, "there is a low possibility of North Korea going ahead with a nuclear test, which could end up being an act of suicide."

There is the possibility of even further economic sanctions, such as suspension of petroleum imports, should North Korea continue with military provocations.

According to sources knowledgeable about China-South Korea ties, similar activity around the Punggyeri site was observed in April.

China took those preparations to be a sign that North Korea was about to conduct a nuclear test. Chinese officials explained to their counterparts in the United States and South Korea that pressure was applied at that time on North Korea in the form of a temporary suspension of Air China flights between Beijing and Pyongyang.

Meanwhile, North Korea has shown little sign of being cowed by the sanctions.

On May 1, the North Korean Foreign Ministry issued a statement under the name of a ministry spokesperson that indicated a sixth nuclear test was imminent. It said North Korean leadership in the person of Kim Jong Un would decide the time and place for nuclear tests that would be conducted "repeatedly and continuously."

In another apparent provocation, the online version of Rodong Sinmun, the official newspaper of the Workers' Party of Korea, said in an article on June 10 that, "The timing of the test firing of an intercontinental ballistic missile is not far away."

With the exception of what appeared to be an ICBM, all new types of missiles that were displayed at an April 15 military parade in Pyongyang have been fired.

The source knowledgeable about North Korea said, "There is the possibility North Korean leadership has determined that the environment to proceed with a nuclear test has been established."

For one thing, preparations do not appear to have been completed between Japan, the United States and South Korea for the transport of strategic materials and evacuation of Japanese nationals in South Korea in the event of a military conflict on the Korean Peninsula.

The Trump administration continues to be rocked by allegations about Russian involvement in last year's U.S. presidential election and friction has also emerged between Seoul and Washington over the deployment of the U.S. military's Terminal High Altitude Area Defense (THAAD) system in South Korea.