Last month, an aid worker in Myanmar named Khon Ja flipped through her Facebook feed and noticed something was missing. Something important.

A page run by a rebel group that controls a stretch of jungle-covered territory near China’s border, known as the Ta’ang National Liberation Army, had disappeared.

Its reports on the location and severity of fighting no longer appeared on Facebook. Neither did its posts on the welfare of civilians under its control.

Ms. Khon Ja, who works to protect civilians from the ravages of Myanmar’s decades-long civil war, had come to rely on the Facebook page, which might reveal, say, battle lines edging toward a village.

“Sometimes we need to evacuate people,” she said. Other times, her team could at least warn local families to take cover, and send money for blankets and medicine.

Reporters are largely blocked from conflict areas in Myanmar, and government accounts are considered unreliable. The rebels’ accounts, too, had to be read with a skeptical eye, but did offer valuable, real-time updates of what was happening on the ground.

Like many in Myanmar, the rebels communicated with the public largely through Facebook — until the social network said no more.

When Ms. Khon Ja tried to find out why the Ta’ang National Liberation Army had gone silent, she learned that it was among four rebel groups in Myanmar that Facebook had decided to ban. Not only did it take down a host of pages, but it also prohibited users from posting words of praise or support for the rebels.

“We don’t want the platform to be used to normalize or further the goals of these groups,” said Rafael Frankel, Facebook’s policy director for Southeast Asia.

Mr. Frankel cited reports that the guerrillas had attacked civilians, backed up by independent rights groups, and said removing their posts was meant to curb their ability to organize or encourage violence.

The bans are the latest sign of Facebook’s growing power in many countries, where its control over the flow of information and public discourse makes it akin to national broadcaster, public utility and political regulator all rolled into one.

And they are the latest test of Facebook’s ability to wield that power responsibly.

But Facebook’s intervention has brought unintended consequences that have troubled humanitarian groups and conflict experts in Myanmar.

The bans, they say, have swept up pages crucial for protecting some of the country’s most vulnerable communities. And some worry that they will further tilt control of information about the conflict toward the military, which rights groups consider to be guilty of far graver atrocities than the rebels.

Senator Edward J. Markey, Democrat of Massachusetts, sent a letter to Facebook saying he was “deeply troubled” by the bans. He called the groups’ Facebook presence “important to broader efforts at a lasting peace” and warned that their removal would disrupt efforts by humanitarian groups and by international conflict observers, “including the United States.”

Ms. Khon Ja, crediting the platform’s reach in Myanmar’s conflict zones with helping to save lives, said, “Facebook is very important for us.” Without pages like that of the Ta’ang rebels, she worried, it could become harder to shield civilians from violence.

But Arakan Army officers had used Facebook’s messenger service to coordinate with aid groups, a now-closed line of communication that Ms. Khon Ja called crucial for steps like reuniting former child soldiers with their parents.

But of all the pages Facebook removed, none alarmed activists as much as the closing of Laiza TV. It is mostly known as a radio station and video-filled Facebook page, both widely followed in Myanmar’s Kachin State.

Many members of the Kachin ethnic minority support the Kachin Independence Army, and Laiza tends to do so, also. Facebook said that this was why it removed the page.

But Laiza also conveyed day-to-day news from the war-torn region, where government accounts are unreliable and most media cannot go.

Gum San Nsang, the president of the Kachin Alliance, a diaspora group based in Washington, said that the Facebook ban had cut off information about the tens of thousands of Kachin who, as a result of the fighting, live in camps and rely on groups abroad for help.

“We don’t know what’s happening, we don’t know the latest on displaced people, we don’t even know the weather,” he said, referring to the tendency of floods or landslides to displace vulnerable communities. “Facebook is the only thing we have to keep in touch.”

A battle or disease outbreak might prompt a torrent of donations, but only if the news can get out. Outlets like Laiza rely on Facebook because it is the only online tool that is both widely available and cannot easily be hacked by the government.