The Army special operations soldiers arrived in December to help Saudi counterparts locate launch sites and destroy the Houthis’ missile supplies, according to the Times, which cited US officials and European diplomats.
Citing operational security, the Pentagon said it could not comment on the makeup of forward-deployed forces.
The Pentagon’s “limited non-combat support, such as intelligence sharing, focuses on assisting our partners in securing their borders from cross-border attacks from the Houthis,” military spokesman Major Adrian Rankine-Galloway said.
The Times said there was no indication the commandos had crossed into Yemen.
The unannounced move shows a deepening US involvement in Yemen’s war that has seen the country spiral toward famine and claimed almost 10,000 lives.
Since March 2015, Saudi Arabia has led a US-backed coalition of Arab states fighting to roll back the Houthis in Yemen and restore its neighbour’s internationally recognised government to power.
Officials told the Times the US troops are training Saudi forces to secure the border, which has seen an increase of Houthi ballistic missiles cross into the kingdom in recent months.
The Saudi troops are also working closely with US intelligence experts in the southern Saudi city of Najran, the Times said.
The Houthis, who hail from northern Yemen, control Sanaa and much of the country’s north – which borders Saudi Arabia – and the key Hodeida port on the Red Sea coast.
US lawmakers have sounded growing alarm about America’s support for the Saudis in Yemen, while President Donald Trump has bolstered ties with Riyadh and fostered a close relationship with Saudi Arabia’s Crown Prince Mohammed bin Salman.
Separately from Yemen’s civil war, the Pentagon has been bombing al-Qaeda in the Arabian Peninsula for several years, and has sent in ground troops to conduct raids against the jihadists.
The campaign against AQAP, which has taken advantage of the war to expand its presence in several areas to the south and east, has intensified under Trump.
Civilian casualties from coalition airstrikes have drawn criticism from rights groups, and in October the United Nations placed the Saudi alliance on a “blacklist” for killing and maiming children.
Along with its air campaign, the Saudi-led coalition has imposed periodic blockades on Yemen’s ports. Both actions have killed more than 10,000 people – most of them civilians – and have left more than 18 million in need of aid, according to the UN.
The Saudi-led coalition’s war against Houthi rebels has led to the “world’s worst humanitarian crisis”, the UN said.