Hisham al-Omeisy, pictured earlier in August. (Australian Broadcasting Corp.)

In Yemen’s often-ignored war, Hisham al-Omeisy has been one of its most visible documenters. He has used social media to provide the world a window into the nation’s deepening humanitarian crisis and criticized all sides of the conflict.

On Monday, security agents with Yemen’s rebel Houthi movement arrested Omeisy in the capital, Sanaa, and they are holding him in an undisclosed location without access to a lawyer or his family, two international human rights groups said Friday in statements.

“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,” Samah Hadid, director of Amnesty International’s advocacy campaigns in the Middle East, said in statement.

In recent months, dozens of activists, journalists, lawyers and others critical of the rebel Houthi government and its partner, former president Ali Abdullah Saleh, have been arbitrarily arrested or have vanished into the security apparatus, human rights groups say. Some have been tortured or have died in custody, the groups say.

What makes Omeisy’s detention surprising is that he was mostly critical of the Saudi-led coalition backing the government of President Abed Rabbo Mansour Hadi, who is trying to oust the rebels from Sanaa and much of the north. The coalition has launched numerous airstrikes that have killed thousands of civilians, including children, according to the United Nations.

Omeisy, 38, frequently tweeted in English and Arabic and regularly sent pictures and video to his nearly 24,000 followers. The warring sides have sought to restrict visas and access to flights for Western journalists to cover Yemen’s conflict, so activists such as Omeisy have become important voices illuminating the unfolding tragedy.

“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,” said Sarah Leah Whitson, Middle East director at Human Rights Watch.

Yemen, the Middle East’s poorest country, has been besieged since March 2015 after the Houthi overthrow of the Hadi government. That prompted Saudi Arabia and other regional Sunni Muslim powers to enter into the conflict to restore Hadi to power. The Shiite Houthis are widely believed to be backed by Iran, Saudi Arabia’s regional rival.

More than 10,000 civilians have died, and some 3 million people have been displaced from their homes, according to the U.N. humanitarian-affairs office. A deepening hunger crisis is propelling the country to the brink of famine, and a cholera outbreak is devastating the population.

Omeisy, a political activist who gained attention during Yemen’s Arab Spring revolts that ousted Saleh five years ago, tweeted multiple times a day, and his comments were often scathing. In one, four days before his arrest, he wrote: “Sana’a airport closure by Saudi-led coalition & preventing medical evac, killed more people than airstrikes. #Yemen

It is unclear why Omeisy was targeted. Two days before his arrest, he wrote in a tweet: “Armed goons backed by corrupt officials are forcefully taking real estate properties in Sana’a. They just showed up at my door. #Yemen

On Monday, Omeisy was arrested by 15 security officers about 2:45 p.m. and was being held by the country’s National Security Bureau, said human rights 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,” Hadid said.