South Korea is closely watching North Korea over the possibility it may launch another intercontinental ballistic missile as soon as Saturday, as it celebrates its founding anniversary.
Seoul's Unification Ministry said Pyongyang could potentially conduct its next ICBM tests this weekend, or around October 10, another North Korean holiday marking the founding of its ruling party.
North Korea has previously marked key dates with displays of military power, but now its tests appear to be driven by the need to improve missile capabilities.
The North carried out its sixth and most powerful nuclear test to date on Sunday in what it claimed was a detonation of a thermonuclear weapon built for its ICBMs.
The country tested its developmental Hwasong-14 ICBMs twice in July and analysts said the flight data from the launches indicate the missiles could cover a broad area of the continental United States, including major cities such as Los Angeles and Chicago, when perfected.
North Korea fired the ICBMs at highly lofted angles in July to reduce ranges and avoid other countries, but South Korean officials said the next launches could be conducted at angles close to operational as the North would test whether the warheads survive the harsh conditions of atmospheric re-entry and detonate properly.
President Donald Trump reiterated on Thursday that US military action is "certainly" an option against North Korea, as his administration tentatively concurred with the pariah nation's claim to have tested a hydrogen bomb.
A senior administration official said the US was still assessing last weekend's underground explosion.
"Military action would certainly be an option," Mr Trump told a White House news conference. "I would prefer not going the route of the military, but it's something certainly that could happen."
He added: "I don't put my negotiations on the table, unlike past administrations. I don't talk about them. But I can tell you North Korea is behaving badly and it's got to stop."
North Korea broke from its pattern of lofted launches last month when it fired a powerful new intermediate range missile, the Hwasong-12, over northern Japan.
North Korean leader Kim Jong Un then called the launch a "meaningful prelude" to containing the US Pacific island territory of Guam and called for his military to conduct more ballistic missile launches targeting the Pacific Ocean.
Kim, a third-generation dictator in his 30s, has conducted four of North Korea's six nuclear tests since taking power in 2011.
Washington, Seoul and Tokyo have been pushing for stronger sanctions to punish Pyongyang over its nuclear activities, such as denying the country oil supplies. China and Russia have been calling for talks, saying sanctions are not working.