South Korean intelligence officials said Wednesday that around 20 Scud missiles fired at Saudi Arabia from Yemen by Houthi rebels and their allies originated in North Korea.

"North Korea has sold missiles to Yemen and sent missile engineers to that country in the 1990s," a former North Korean intelligence official told South Korea's Yonhap News Agency.

The Shia rebels fired the Scud missiles in retaliation for an ongoing bombing campaign in Yemen by a Saudi-led coalition.

An independent expert told VICE News the South Korean report about the North Korean origins of the missiles is likely accurate. "Back in 2002, Yemen purchased around 20 Scuds from the North Koreans," Joseph Bermudez, an arms expert with All Source Analysis, said. "So it's likely the Scuds being used in the conflict did come from North Korea originally."

Scud missiles are relatively cheap ballistic missiles first developed by the Soviet Union in the 1960s. North Korea is known to manufacture and export Scuds to developing countries, including Egypt, Iran, Myanmar, and Yemen, according to Bermudez. Over the last decade, the US has made an effort to clamp down on the circulation of Scuds, but dozens of North Korean missiles are still thought to be in Yemen.

"This is not a surprise," Joel Wit, a North Korea expert at Johns Hopkins University, explained to VICE News. "Yemen has been a customer of North Korea for some time."

Since the Saudi-led airstrikes began in late March, the coalition has imposed a de facto blockade on Yemen, obstructing even shipments of humanitarian aid to many parts of the country. So far, the Saudi-led bombings have killed 1,895 civilians and injured an additional 4,182 people, according to the latest figures from the United Nations.

Human rights groups have accused both sides in the conflict of war crimes.

According to Reuters, the Houthis first launched a Scud missile in early June, after two months of Saudi-led bombing. Saudi Patriot missiles intercepted the Scud, which was aimed at an airbase.

Few of the 20 Scuds fired by Houthis over the past two months have actually landed in Saudi Arabia. The Saudis claim to have intercepted around 40 percent of the missiles fired across their border, though the number is impossible to verify.