One of Yemen’s most prominent political analysts and activists is being held incommunicado, without access to a lawyer or his family, after he was detained in Sana’a by the Houthi rebels that control the city.

Hisham al-Omeisy has been a rare on-the-ground commentator during Yemen’s civil war, and has been critical of both sides in his outspoken Twitter posts.

He is understood to have been detained on 14 August in Sana’a’s Jawlat al-Misbah neighbourhood.

The Iran-backed Houthis are allied with Yemen’s former president Ali Abdullah Saleh, and have been in conflict with forces loyal to another president, the ousted Abd-Rabbu Mansour Hadi, since 2015.

A military intervention led by Saudi Arabia aimed at reinstating Hadi has exacerbated a dire humanitarian situation.

Omeisy’s last post on Twitter indicated that “armed” forces had showed up at his doorstep. He is now being held in an undisclosed location.

Amnesty International criticised what it described as Omeisy’s arbitrary detention. “Hisham al-Omeisy has been detained without charge or a court appearance in breach of Yemen’s constitution, which requires anybody arrested to be presented in court within 24 hours,” said Samah Hadid, Amnesty’s director of campaigns in the Middle East.

“This detention illustrates the lengths to which local Houthi-Saleh authorities are willing to go to silence peaceful activists. Hisham al-Omeisy is a prisoner of conscience, whose only ‘crime’ is peacefully exercising his right to freedom of expression, and he must be released immediately.”

Omeisy, 38, has provided crucial insights into the war in Yemen, where international journalists have been prevented from reporting on the ground.

The country is facing the world’s largest humanitarian crisis, according to the UN. An outbreak of cholera has slowed since June but is still the world’s worst.

Speaking to the Guardian last year, Omeisy described the situation in Yemen as “Syria on steroids”, warning that the country was on a fast lane to disintegrating as a state, with al-Qaida in the Arabian Peninsula and Islamic State exploiting the power vacuum to expand their foothold in the growing number of ungoverned areas.

When asked who he blamed, he replied: “[I] will need more than 1,000 words to begin answering that question as one will have to deconstruct multiple incidents over a period of the past few years and correctly assign blame to parties … [a] domino effect of a series of missteps and unfortunate events led us to where we are today.

“But I will say this, though Houthis are guilty of hijacking and mismanaging the state, it is the Saudi-led coalition that held the whole nation hostage to the current conflict.”

Human Rights Watch’s Middle East director, Sarah Leah Whitson, echoed Amnesty’s concerns about Omeisy’s detention. “Yemen more than ever needs activists like Hisham al-Omeisy to bring attention to the devastation that war, famine and disease have wrought on the country and its people,” she said.

HRW said both parties in the conflict had severely restricted the access of rights’ groups on the ground. The warring sides have also been uncooperative with most international journalists, preventing them from travelling to report from the field.

The UK has been chided for selling arms to the Saudis despite the high casualty rate from the Saudi-led air campaign over Yemen. Dozens were killed last week when an airstrike hit a hotel on the outskirts of Sana’a.

This week it emerged that Saudi Arabia had embarked on a charm offensive in New York to stop its name being added to a UN list of armies deemed responsible for killing and maiming children. The Guardian