Security is surprisingly easy to negotiate, even compared to other government agencies, though once inside, stern-faced, suited men with microphones in their ears provide a watchful presence. Smartphones and cameras are not permitted.

Banners reading "For the Freedom; For the Nation; For the People," hang from the ceiling; a poster depicts an attractive woman toting a huge handgun and journalists are invited to fire revolvers at the agency's in-house shooting range.

This being modern South Korea, the lobby of the headquarters features a cheery coffee shop, and NIS activities extend to publishing investment guides to Uzbekistan and Azerbaijan.

But upstairs a museum - visit by appointment only - makes clear the threat posed by North Korea, a country less than 80km north of the complex.

Exhibits include two spy drones that crashed on South Korean territory: one has been part dismantled, revealing a commercial digital camera embedded in its fuselage.

Weapons captured from North Korean infiltrators over the years are displayed: Kalashnikov assault rifles, submachine guns, miniature pistols with silencers and some that seem to have been taken from a James Bond set - faux pens containing bullets and toxic needles for close-range kills.

The NIS is powerful, covering both internal and external intelligence. It is essentially a combined MI5-MI6, or CIA-FBI, under one roof. It descends from the Korean CIA, or KCIA, founded under President Park Chung-hee - the general who seized power in a 1961 coup, engineered South Korea's "economic miracle" and ruled with a rod of iron until his assassination in 1979. He is also father of the country's current president, Park Geun-hye.

The KCIA was notorious. Its interrogation centre on Mount Namsan in central Seoul was feared by anti-government activists. Its entire lower floor was devoted to bathrooms.

In its most infamous operation in 1973, it kidnapped then-opposition leader Kim Dae-jung and was preparing to drown him in the Sea of Japan when the US CIA intervened, saving the life of a future Nobel Peace Prize winner and president.

The agency was reformed after the country's 1987 democratisation, but in recent years, has been embroiled in controversies again.

NIS agents were found to have meddled in the 2012 presidential election in the form of an online smear campaign targeting the opponent of the eventual winner, conservative Park Geun-hye. Last year, Won Sei-hoon, the agency's then-director general, was found guilty of overseeing the operation and sentenced to two years in jail.

Then, last May, Won's successor lost his job and two NIS agents were jailed after the latter were found to have forged documents to frame a defector as a North Korean spy.

The agency was lambasted in an August 2014 report by the International Crisis Group for intelligence failures, politicisation of intelligence and intervention in domestic politics.

The group urged greater oversight and restrictions on cyber operations involving the North.

"It shows a lack of institutional checks and balances and that, to me, is the most troubling thing," said Jack Burton, a columnist. "There is too much executive power in this country."

NIS officials declined to discuss these issues yesterday. Briefings were off-the-record, controversies were not mentioned and officials did not take questions.

The ups and downs of South Korea's spy agency

Anonymous dedication to freedom and truth

Founded as Korean CIA

Hotline established to report North Korean spies

North Korean spy ring uncovered in Berlin

Aborted kidnapping of opposition politician Kim Dae-jung

Spy ring captured on South Korean island of Ulleung-do

North Korean bomber of KAL flight 858 apprehended

Agency reorganised and rebranded as NIS

National Cyber Security Command established

North Korean assassin sent to kill high-ranking defector held

North Korean assassin tasked with killing propaganda balloon defector apprehended

Agency head found guilty of overseeing online campaign to smear leftist presidential candidate