Saudi Arabia has violated international humanitarian law in Yemen and unjustly jailed activists peacefully seeking social reforms, a human rights group said on Thursday.

The Saudi-led military coalition bombing Houthi rebels in Yemen carried 87 unlawful attacks resulting in nearly 1,000 civilian deaths, New York-based Human Rights Watch said in a new report.

The Houthis control most of northern Yemen, and the Saudi alliance - supported by the United States and United Kingdom - launched military operations in March 2015 to drive them out.

The war has decimated the Arab world's poorest nation with more than 10,000 people killed, disease proliferating, and millions facing famine.

The Saudi-led coalition has denied war crime allegations.

Back at home, more than a dozen political activists have been convicted on "vague charges arising from their peaceful activities" and were slapped with lengthy prison sentences, HRW said.

Powerful Crown Prince Mohammed bin Salman has said he wants to modernise the kingdom and introduce social reforms. Women were allowed to attend a men's football match for the first time last week, and a decades-long ban on watching films at cinemas was lifted this week.

But the US rights group said the moves weren't enough.

"Mohammad bin Salman's well-funded image as a reformist falls flat in the face of Yemen's humanitarian catastrophe and scores of activists and political dissidents languishing in Saudi prisons on spurious charges," said Sarah Leah Whitson, Middle East director at Human Rights Watch.

"Baby steps on women's rights reforms don't paper over Saudi Arabia's systemic abuses."