Ethiopia says its military has taken control of capital in defiant Tigray region

Hours later, the country's army chief of staff said government forces had taken full control of Mekele - home to at least half a million people - and Prime Minister Abiy Ahmed claimed in a separate statement that the "last phase" of the operation against leaders of the Tigray People's Liberation Front (TPLF) had been completed.

A communications shutdown and bans on media access across most of Tigray has made verifying the government's claims near impossible. Fighting began Nov. 3 and has been marked by reports of mass atrocities and a deepening humanitarian crisis in an already stressed region.

The offensive followed the end of a 72-hour ultimatum Abiy issued to the rebelling leaders of the Tigray region to lay down arms after three weeks of unrest that sent more than 40,000 civilians fleeing across the border into Sudan.

The military campaign is directed against the TPLF, the political party in charge of the province and which once ruled a coalition that led Ethiopia for nearly three decades. After Abiy's rise, leaders returned to their power base in Tigray. Relations with the central government in recent months have imploded.

Watch: What's behind the conflict in Ethiopia?

Abiy has rejected international calls for talks with the TPLF, saying dialogue would "nurture a culture of impunity with devastating cost to the survival of the country." Ethiopia is made up of ten semiautonomous ethnolinguistic regions.

Abiy won last year's Nobel Peace Prize for ending a two-decade-long standoff with neighboring Eritrea.

Abiy's statement said that the Ethiopian military had retaken Mekele airport and a major command post in the city, freed thousands of soldiers that had been "held hostage" by the TPLF, and done so with precision to avoid civilian casualties. Tigrayan officials, however, noted heavily populated areas of Mekele were being shelled.

Ethiopian forces "started hitting with heavy weaponry and artillery the center of Mekelle," the local government said in a statement to Tigrayan media. Video of the claim also surfaced on Facebook, and two humanitarian groups with staff in the city confirmed the reports to Agence France-Presse.

A senior diplomat said blasts rocked the city's north on Saturday, citing satellite phone calls from a government official who heard them. Local television, which had been operating until midday, suddenly went static.

"It's happening," said the diplomat, who was not authorized to discuss the matter publicly.

The shells appeared to be coming from surrounding areas, said a humanitarian official in Addis Ababa, who fielded reports from staffers in Mekele. No one could see federal forces.

International concern had mounted that Mekele's half a million residents could get caught in the violence. Aid workers said the exodus from Tigray included pregnant women, children and the ill.

After fleeing Tigray, refugees in Sudan recount brutal killings

Billene Seyoum, a spokeswoman for the prime minister's office, denied claims that Ethiopian forces were striking civilian areas. "The safety of Ethiopians in Makelle and Tigray region continues as priority for the federal government," she said.

The three African Union envoys who met with Abiy - former Presidents Joaquim Chissano of Mozambique, Ellen Johnson Sirleaf of Liberia and Kgalema Motlanthe of South Africa - urged the Ethiopian leader this week to open dialogue with the rebelling TPLF.

He has publicly refused to take that step, calling the TPLF an illegal "clique" and an internal matter for Ethiopia to resolve on its own. (He barred the envoys from seeing TPLF commanders.)

Pope Francis was among those appealing for peace Saturday.

"I invite everyone to pray for #Ethiopia where armed clashes have intensified and are causing a serious humanitarian situation," he wrote on Twitter. "I appeal to the parties in conflict so that the violence might ceases, life may be safeguarded and the populations can regain #peace."

The U.N. refugee agency has warned that a "full-scale humanitarian crisis" is unfolding in Tigray. Food and water may be running out in the province of 6 million, putting untold lives at risk.

Aid workers said they had no way to reach people who had been forced from their homes for weeks. Access remains unclear, several groups said, after Ethiopian officials announced the opening of humanitarian routes on Thursday.

Abiy's government has blamed the rebels in the north for the crisis that has grabbed the world's attention.

"The last peaceful gate which had remained open for the TPLF clique to walk through has now been firmly closed as a result of TPLF's contempt for the people of Ethiopia," the prime minister's office said in a statement.

The most senior State Department official on African affairs, Tibor Nagy, tweeted on Saturday that "fighting and shelling in the Mekele area are a very grave concern. We urge an immediate end to conflict and restoration of peace in Tigray. Civilians must be protected and humanitarian access provided to aid those in need."

President-elect Joe Biden's national security adviser nominee, Jake Sullivan, urged dialogue led by the African Union.

"I'm deeply concerned about the risk of violence against civilians," he said on Twitter, "including potential war crimes."