Saudi Arabia has begun moving military equipment including artillery to its border with Yemen, raising fears that it could be drawn into the country's escalating civil war.
The move comes as the Iranian-backed Houthis, the country's main rebel group who took control of Yemen's capital Sanaa last year, made a significant advancement south.
"The conflict risks spiralling into a proxy war," with Shiite Iran backing the Houthis, and Saudi Arabia and the other regional Sunni Muslim monarchies backing ousted president Abed Rabbo Mansour Hadi, reports Reuters.
Saudi Arabia says it will not allow Tehran to "sow sectarian strife in the region" and has vowed to support and protect Hadi.
Security sources in Washington say it is unclear whether the Saudi weapons are offensive or defensive. One source described the size of the military build-up as "significant" and said Saudi Arabia could launch airstrikes to defend Hadi if his stronghold in the southern city of Aden comes under attack.
