ADDIS ABABA, Ethiopia — Ethiopian Prime Minister Abiy Ahmed sent troops into the northern Tigray province on Wednesday and declared a "military confrontation" after the regional government attacked a federal military base there.

In a statement, Abiy’s office accused the Tigray People’s Liberation Front (TPLF), the renegade province’s ruling party, of attempting to steal artillery and other military equipment during the attack.

“The last red line has been crossed with this morning’s attacks and the federal government is therefore forced into a military confrontation,” Abiy’s office said in the statement, citing what it called months of provocation and incitement. It said the mission was to “save the country and the region from spiraling instability.”

Fighting in the continent’s second-most-populous country could destabilize the entire sensitive Horn of Africa region. Citing diplomatic sources, Reuters reported heavy fighting in the mountainous province.

International pressure began to mount amid concerns that civilians could get caught in the middle of the fighting. The U.S. Embassy in Addis Ababa called for an “immediate de-escalation of the current situation in Tigray.” Britain also called for restraint and issued an updated advisory to travelers to exercise extreme caution in Tigray and the neighboring northern Amhara region amid reports of clashes, it said.

Ethiopia’s Council of Ministers declared a six-month state of emergency in Tigray, asserting federal authority over the region. The TPLF once dominated Ethiopia’s ruling coalition before Abiy took office in 2018 and the party was sidelined.

Tensions with Tigray escalated in recent months after the region went ahead with its own local elections even though all polls were suspended because of the coronavirus pandemic. On Oct. 7, Ethiopian lawmakers voted to withhold budget support from Tigray, a move that one Tigrayan official said was “tantamount to a declaration of war.”

Most of Ethiopia’s military equipment is in Tigray because of a long-running war against neighboring Eritrea, which ended in 2018, when the countries signed a peace deal. Now, Abiy, who won the 2019 Nobel Peace Prize for his reform efforts in Ethiopia and peace overtures to Eritrea, is trying to dislodge the Tigrayan-linked old guard elements from the military.

A statement on Tigray TV attributed to the regional government accused the federal government of deploying troops to “invade Tigray to cow the people of Tigray into submission by force,” the Associated Press reported. It said the regional government had started to oppose the federal government to avert more destructive measures.” It warned of “proportional measures” for any damage to people or property.

“This war is the worst possible outcome of tensions that have been brewing,” said William Davison, a senior analyst at the International Crisis Group.

“Given Tigray’s relatively strong security position, the conflict may well be protracted and disastrous,” he said. A war could “seriously strain an Ethiopian state already buffeted by multiple grave political challenges and could send shock waves into the Horn of Africa region and beyond,” he added.

Conflict in Tigray would further increase instability in Ethiopia and add to a host of recent crises that Abiy’s government has faced, including tensions with Egypt over an Ethiopian dam project on the Blue Nile and a locust outbreak across East Africa.

As the country has opened and political reforms have taken hold, ethnic and other political violence has also flared up. At least 54 people from the ethnic Amhara group were killed in a schoolyard by rebels Sunday, according to Amnesty International. The government blamed the Oromo Liberation Army for the attack in the far western part of Oromia near the border with South Sudan.

Tigrayans dominated Ethiopian politics after helping to oust a Marxist government in 1991, but their influence has waned under Abiy, and they quit his ruling coalition last year. Tigray’s population makes up about 7 percent of Ethiopia’s 108 million people but has a well-armed regional paramilitary led by former national army generals and is wealthier than most regions.

Wroughton reported from Cape Town, South Africa.