Donald Trump has expressed openness to a future meeting with North Korean ruler Kim Jong-Un
A US anti-missile defence system deployed in South Korea is now operational and can defend against North Korean missiles, Seoul has said.
The Terminal High Altitude Area Defence interceptor system (THAAD) has "early capability" to respond to North Korea's nuclear and missile threat, a Defence Ministry spokesman said.
South Korean officials, however, told the Reuters news agency it would not be fully operational for some months.
The system, set up at a converted golf course in Seongju, in the country's southeast, has sparked controversy.
China sees the system as a threat, and experts have debated whether its security benefits would outweigh possible drawbacks if relations with Beijing worsen.
Some are also angry at US President Donald Trump's statement he would make Seoul pay $1bn for it.
And Seongju residents fear North Korea may target their town and are worried about the rumoured health hazards linked to system's powerful radar.
The favourite to win South Korea's presidential election on 9 May, Moon Jae-in, a liberal who calls for engagement with North Korea, has said he would reconsider THAAD if he became president.
Tensions remain high on the Korean peninsula.
Mr Trump says he is not ruling out military action against North Korea - though he has also expressed openness to a future meeting with its ruler Kim Jong-Un.
"If it would be appropriate for me to meet with him, I would absolutely, I would be honoured to do it," Mr Trump told Bloomberg News.
In an interview with CBS' Face The Nation, shown on Sunday, Mr Trump said Kim was obviously a "pretty smart cookie" as he took power in his 20s and held it despite "a lot of people" trying to take it away.
On Saturday, North Korea conducted a missile test which reportedly failed soon after launch.
Its Foreign Ministry said on Monday the country will speed up measures to bolster its nuclear programme "at the maximum pace."
The North conducted two nuclear tests last year, which experts said would have improved its knowledge for making nuclear warheads small enough to fit on missiles.