NAIROBI, Kenya — He swept to power preaching unity and hope, struck a landmark peace deal with the longtime foe Eritrea, released thousands of political prisoners, lifted restrictions on the press and promised to overturn decades of repressive authoritarian rule. For those accomplishments, Prime Minister Abiy Ahmed of Ethiopia was awarded the Nobel Peace Prize in 2019.

But now, mired in a grinding civil war, Mr. Abiy has embarked on a radically different track, stoking war fever and urging all able-bodied men and women to join a widening military campaign, either as combatants or in support roles.

The Ministry of Defense has not said how many new recruits it has signed up, but the spokesman for Sintayehu Abate, deputy mayor of Addis Ababa, the capital, has said that 3,000 residents of the city have enlisted since the campaign started and that thousands more have reportedly signed up around the country.

Critics have denounced Mr. Abiy’s latest campaign, saying the injection of fresh recruits into the fighting will only lead to more bloodshed in the deeply polarized and ethnically divided nation, potentially destabilizing the wider Horn of Africa.

Ethiopia is a patchwork of at least 80 ethnic groups and 10 regional governments. Analysts worry that a protracted conflict could push groups within Ethiopia to take sides and potentially draw in countries from across the region.

“This is a declaration to turn civilians into combatants that will further plunge the country into a genocidal war and create bad blood between peoples for generations to come and an economic free-fall,” said Mehari Taddele Maru, a professor of governance and geopolitics at the European University Institute.

Over the past nine months of conflict, thousands of people have been killed and some two million have been displaced, while hundreds of thousands of others face famine conditions amid reports of massacres, sexual assault and ethnic cleansing.

The roots of the conflict can be traced to last November, when Mr. Abiy ordered a military offensive against the Tigray People’s Liberation Front, onetime rebels who led Ethiopia with an iron fist from 1991 until Mr. Abiy’s ascent in 2018. He accused the group of attacking a federal military base and trying to steal weapons.

The war quickly escalated, with militia fighters from the Amhara region to the south and Eritrean troops from the north joining the Ethiopian military against the Tigrayan forces.

But the swift victory Mr. Abiy promised never materialized. Instead, the hostilities settled into a grinding war in different pockets of Tigray. In June, Mr. Abiy declared a unilateral cease-fire after the rebels shockingly routed the government forces and captured Mekelle, the regional capital of Tigray, altering the course of the war.

Emboldened by their wins, the Tigrayans issued a set of demands that called, among other things, for a “transitional arrangement” that would essentially see Mr. Abiy removed from power.

Mr. Abiy rejected those demands and recently urged Ethiopians at home and abroad to defend the “motherland” and be “the eyes and ears of the country in order to track down and expose spies and agents” of the Tigrayan forces.

Since then, the Ethiopian authorities have ramped up mass recruitment drives, calling on popular musicians and artists to galvanize the war effort.

This past week, the military posted photos from the town of Debark in the northern Amhara region where young men — wielding machetes, guns and sticks studded with nails — rallied in support of the war and enlisted in droves. In the eastern city of Jigjiga, hundreds of men, women and some children attended a rally to support government forces.

Among them was Alem Bilatte, 54, a retired army officer, who called the Tigray forces Ethiopia’s “enemy” and promised to train new recruits or go to the battlefront himself. “My blood is boiling,” he said as he registered in the capital.

Bekelech Ayalew, 47, a former infantry nurse draped with an Ethiopian flag, said she was ready to treat soldiers on the front lines. “Sacrificing my blood and dying for Ethiopia is a privilege,” she said.

As the recruitment drive has gotten underway, rebel forces have continued to advance in western Tigray, an area that ethnic Amharas historically claim as their own and took over in the early stages of the conflict. Heavy fighting, including artillery fire, has been reported in the Amhara, Oromia and Afar regions, according to an internal United Nations security document seen by The New York Times.

The dynamics of the war are also shifting as the fighting escalates.

This month the Oromo Liberation Army, designated a terrorist organization by the Ethiopian government, declared an alliance with the Tigrayan forces, raising the prospect of other splinter groups or regional governments becoming involved in the fighting.

Mustafa Omer, the president of the eastern Somali region, which has sent hundreds of soldiers to join the war on the government side, said he would never negotiate with the T.P.L.F., which he said had tortured and killed his brother and made other family members disappear in its authoritarian, nearly three-decade time in power.

“They caused a lot of harm, and they are looking to bring back the same political designs if they win,” Mr. Omer said in a phone interview. “They are a danger to the country.”

Over 200 people, including more than 100 children, who were sheltering at a school and health facility in the Afar region were reportedly killed this month, UNICEF’s executive director, Henrietta Fore, said in a statement. The government and Tigrayan forces have both blamed each other.

The United Nations has said physical access to Tigray remains limited because of lack of infrastructure, floods and security concerns. Underscoring the grim conditions, Samantha Power, the head of the U.S. Agency for International Development, said that aid workers could run out of food to distribute as soon as this week.

UNICEF said that almost half of all pregnant women and breastfeeding mothers screened in Tigray were acutely malnourished, while the number of malnourished children was expected to jump tenfold in the next year.

A recent flare-up in ethnic violence — particularly between ethnic Afar and Somalis — has added to the country’s growing security woes. Human Rights Watch said on Wednesday that ethnic Tigrayans were being arbitrarily detained and disappeared in the capital — adding to discriminatory practices that many Tigrayans reported from the onset of the war.

Until recently the most stable of the countries in the Horn of Africa, Ethiopia and its mounting domestic crisis pose a significant risk to the region’s overall stability and economic growth, said Sanya Suri, the East and Southern Africa analyst at the Economist Intelligence Unit.

A protracted conflict, she said, “sends worrying signs to investors over the government’s efficacy” and would derail Ethiopia’s agenda for overhauling the economy, which included liberalizations and privatization of key sectors like telecommunications.

“Ethiopia’s long-run growth prospects also remain grim,” Ms. Suri said.

Last week, the U.S. special envoy for the Horn of Africa, Jeffrey Feltman, was back in the region in a bid to halt the fighting. In addition, President Recep Tayyip Erdogan of Turkey offered to back a peaceful mediation to the war. But with Mr. Abiy rebuffing a meeting in early August with Ms. Power, Tigrayan forces advancing and fresh government recruits heading to face them, there is little prospect so far for a substantial de-escalation, experts say.

For many Ethiopians, that means bracing for more bloody days ahead.

“The prime minister is calling Ethiopians to fight against their own people,” Weyni Asgedom, 28, a former bookstore owner in Mekelle whose husband is fighting alongside the Tigrayans, said in an interview. “The only choice is to fight back.”