Saudi Arabia's military build-up near its southern border with Yemen is purely defensive, two Saudi sources familiar with the matter told Reuters.

"We are protecting our borders only," said one of the sources when asked if the kingdom planed to intervene in Yemen.

"Tanks have been moved to the border but this is only to defend the country," said the second Saudi source.

The buildup follows a southward advance by Iranian-backed Houthi Shi'ite militants who took control of the capital Sanaa in September and moved closer on Wednesday to Aden, the base of Saudi-supported President Abd-Rabbu Mansour Hadi.