The United States said last month it was watching the Korean peninsula closely after reports that North Korea may be planning a fourth nuclear test since 2006, and it urged Pyongyang not to take any step that would threaten regional peace.
The Comprehensive Test Ban Treaty Organization (CTBTO), set up in the mid-1990s, says it now has more than 85 percent of a planned worldwide network of 337 facilities up and running to check the planet for any atomic bomb testing.
The Vienna-based CTBTO's International Monitoring System consists of stations designed to pick up seismic, radionuclide, infrasound or hydroacoustic signals.
"The normal (mode) of the organisation is being highly on alert," Lassina Zerbo, CTBTO executive secretary, told Reuters when asked about the possibility of a North Korean nuclear test.
"I can assure you that we are always ready to detect anything, anywhere, anytime," he said. "We stand ready to face any type of test or event."
The first, almost immediate indication would likely be in the form of seismic shockwaves, while it could take weeks to pick up any "smoking gun" radioactive traces, if at all.
When North Korea last detonated a nuclear device - in February 2013 - the CTBTO detected radioactive gases that could have come from the explosion only about two months later.
Zerbo said this remained "unique" as it had not been picked up by anybody else, including the five recognised nuclear weapon states - the United States, China, Russia, Britain and France.
"None of those countries were able to detect this sniff of radio-isotope 50 days or so after the test," he said.
The impoverished North, which routinely threatens the United States and South Korea with destruction, warned in March it would not rule out a "new form" of atomic test after the U.N. Security Council condemned the country's launch of a mid-range ballistic missile into the sea east of the Korean peninsula.
"I'm hoping that it is only rumours and that they will not cross that red line in pursuing the fourth nuclear test," Zerbo said. "They have been warned by the international community."
North Korea is already subject to U.N. sanctions over its previous three atomic tests.
TEST BAN NOT YET LAW
Zerbo, a geophysicist from Burkina Faso who took office last year, also called on countries which have yet to ratify the test ban treaty to do so in order for it to finally become international law.
There is widespread international support for the pact, which has been ratified by more than 160 states, but it cannot come into effect because some nuclear powers like the United States and China have not yet taken that step.
Proponents argue U.S. ratification of the pact - which they say has helped to dramatically cut the number of tests since the Cold War with only North Korea setting off an atomic device in this century - could encourage other holdouts to sign on.
But, "any of the eight remaining countries should show leadership," Zerbo said, referring to all those whose ratification is still required, also including India, Pakistan, Israel, Iran, North Korea and Egypt.
He said Israel - assumed to be the Middle East's only nuclear-armed country - could become the next to ratify, saying there had been "positive" signs when he visited it in March.
President Barack Obama's administration argues that the United States no longer needs to conduct nuclear tests. But the Senate needs to approve the treaty, which it rejected in 1999.
Opponents argued at the time that a formal, permanent end to testing could erode the reliability of the U.S. nuclear arsenal and some also questioned whether cheaters could be detected.
U.S. Under Secretary for Arms Control and International Security, Rose Gottemoeller, said last month that despite the treaty's clear merits, "we need time to educate the public and Congress to build support for U.S. ratification".