Image copyright AP Image caption Thousands of people have fled North Korea but high level defections are rare

A senior North Korean military officer who oversaw spying operations has defected, say South Korean officials.

The officer has not been named, but the defence ministry in Seoul said he was a senior colonel in the Reconnaissance General Bureau and left last year.

South Korea's Yonhap news agency quoted a source as saying the colonel was seen as elite by other defectors.

More than 28,000 people have fled North Korea since the end of the Korean War, but high level defections are rare.

Last week, 13 North Koreans who had been working in one of the North's restaurants abroad defected as a group.

Yonhap said a number of senior political figures had defected while working overseas recently.

It quoted government officials as saying this was a sign the leadership of Kim Jong-un was cracking.

'Valuable information'

Defence Ministry spokesman Moon Sang-gyun said the South could not release further information on the colonel.

Image copyright AP Image caption Some defectors become active campaigners against the North Korea leadership

One unnamed official told Yonhap the man was the highest-level military official ever to have defected.

"He is believed to have stated details about the bureau's operations against South Korea to the authorities here," said the official.

The Reconnaissance General Bureau handles intelligence gathering and spying operations, as well as cyber warfare, said Yonhap.

The BBC's Stephen Evans in Seoul said such a figure would likely have valuable information about the workings of Kim Jong-un's government.

How do you leave North Korea?

For most North Koreans it is almost impossible. The borders are heavily guarded and few people have the resources to fund an escape.

Image caption Many previous defectors have escaped across the Yalu River into China

Those who do make it out usually cross the river borders into China. They either lay low to avoid being sent back by China to face severe punishment, or try to reach a third country.

There are many cases of diplomats, athletes, musicians and others defecting and claiming asylum while representing North Korea in other countries. Some border guards have simply walked away from their posts.

Relatives they leave behind are almost certain to face persecution or jail.

Interrogated for playing the wrong tune

How many senior North Koreans have left?

Numbers are not widely available but dozens of senior level officials are thought to have defected in the past few years.

The most high-profile defection to date was Hwang Jang-yop, a politician who was considered the architect of North Korea's policy of "juche", or self-reliance.

He claimed asylum at the South Korean embassy in Beijing while on a work visit in 1997. He died in 2010.

About 29,000 people have defected in total since the 1950s, though numbers have fallen in recent years.

How does South Korea treat defectors?

Any North Korean who makes it to the South enters into a rehabilitation programme and is given an aid package to help them start a new life.

Despite this, many find it hard to adjust.

High-level defectors are questioned closely for valuable information, and to ensure they are not acting as double agents.

South Korea denies North's Korea's accusations that it is enticing people to defect.

The North Korean defectors who want to return home