Here is a timeline of the conflict:

2020: Troops Enter Tigray

Military action begins on November 4, 2020, when Abiy orders response to what he calls a "traitorous" attack on federal army camps in Tigray.

He blames the attack on the region's ruling party, the TPLF, which dominated Ethiopian politics for nearly three decades before he took office in 2018.

'War Crimes'

After 10 days of fighting, the United Nations warns of possible war crimes in Tigray.

Neighbouring Eritrea -- which signed a peace deal with Abiy in 2018 that helped him win the Nobel Peace Prize -- is reported to have sent troops into Tigray to help Ethiopian forces.

Two weeks later government forces take Tigray's capital, Mekele.

On November 28, Abiy declares military operations are "completed" but fighting continues.

2021: 'Ethnic Cleansing'

In February 2021, Amnesty International says Eritrean soldiers killed "hundreds of civilians" in November in the holy city of Axum.

For months, Ethiopia and Eritrea deny the involvement of Eritrean forces although Washington speaks of "ethnic cleansing."

On March 23, Abiy admits the presence of Eritrean troops and officials say they massacred more than 100 civilians in Axum.

Elections are held in June across much of Ethiopia but not in Tigray.

Tigrayans Advance

The rebels mount a shock comeback and retake Mekele in late June, before pushing into the neighboring Amhara and Afar regions.

On July 2, the UN says 400,000 people are on the brink of famine in Tigray.

Abiy is sworn in for a new five-year term on October 4.

Two weeks later, Ethiopian aircraft launch deadly strikes on Tigray.

In late October the Tigrayans claim control of two key cities in Amhara -- only a few hundred kilometers north of Addis Ababa.

Abiy At The Front

On November 2, 2020, Ethiopia declares a nationwide state of emergency.

The following day a joint UN-Ethiopian report says crimes against humanity may have been committed by "all sides."

Abiy arrives on the frontline on November 24 to personally direct the counter-offensive, official media say.

In the first weeks of December, the government says it has recaptured a string of towns, including Lalibela, a UNESCO World Heritage site.

Rebels Withdraw

On December 20, the rebels say they are withdrawing from Amhara and Afar and pulling back to Tigray.

Two days later, the government says its forces will not advance further into Tigray, raising hopes of a possible cooling of the conflict.

As the year ends, the UN says dozens of civilians were killed in Tigray between December 19 and 24 in an "intense series of air attacks."

2022: More Airstrikes

On January 7, 56 people are killed in a drone strike on a displaced persons' camp in Dedebit in northwestern Tigray, according to the rebels.

Aid agencies suspend operations in the area, with the UN saying "the intensification of airstrikes is alarming."

On January 14, the UN puts the death toll for the month at least 108 civilians and says war crimes could have been committed.

On January 25, Tigrayan rebels say they have been "obliged" to resume combat in Afar.

'Indefinite Humanitarian Truce'

The UN estimates that 4.6 million people in Tigray lack access to adequate food.

On March 24, the government declares an "indefinite humanitarian truce" to help hasten the delivery of emergency aid into Tigray.

The next day, Tigrayan rebels agree to a "cessation of hostilities" if access to aid is eased.

On April 1, the UN says the first international aid convoy in three months will soon enter Tigray.