SEOUL (AFP) - South Korea's spy agency believes North Korean leader Kim Jong Un, who recently dropped out of public view for nearly six weeks, had surgery to remove a cyst from his ankle, Yonhap news agency reported Tuesday.

In a briefing for legislators, the National Intelligence Service (NIS) said it also had information that Kim had demoted around 200 military officers and that 50 executions had been carried out in the North so far this year.

Citing ruling and opposition party MPs present at the briefing, Yonhap said the intelligence shared by NIS suggested a cyst had been found in Kim's ankle joint back in May.

Foreign medical experts were flown to Pyongyang to carry out an operation to remove the cyst sometime in September or October, the agency said.

Kim dropped out of sight after attending a music concert with his wife in early September.

His prolonged and unexplained absence fuelled rampant speculation about his health and even rumours of a coup in the nuclear-armed state.

He finally reappeared in mid-October, with state media showing him using a walking stick as he conducted a series on inspection visits to new residential complexes in Pyongyang.

Competing theories for his absence had ranged widely from an extended rest period to a leadership coup, via a long list of possible illnesses and ailments including broken ankles, gout and diabetes.

Kim, a heavy smoker, has shown striking weight gain since coming to power and TV footage just prior to his "disappearance" had shown him walking with a pronounced limp.

The NIS intelligence suggested that Kim's weight problem could hamper his recovery from ankle surgery.

Of the 50 executions carried out this year, the NIS said a number were related to an ongoing purge of those close to Kim's uncle and one-time political mentor Jang Song Thaek.

Jang was executed last December on charges of treason and corruption.

On the demotion of the 200 military officers, the NIS said the apparent reason was under-performance in strategic artillery drills.