Forces loyal to the former ruling party in Ethiopia’s rebel Tigray region have entered the regional capital Mekelle, dealing the harshest blow to the government in Addis Ababa since a gruesome conflict began in November.

“We have taken over Mekelle,” said a member of the Tigrayan People’s Liberation Front, eight months after the start of a civil war that is likely to have killed thousands, displaced millions and triggered fears of a famine.

One resident of Mekelle said “everybody is very happy as the fighters control the city”, with people waving red and yellow Tigrayan flags. But they added that within a radius of about 25km around the city “there is still war”.

Calls to officials in the interim administration in Tigray, which was appointed by Addis Ababa, went unanswered, but eyewitnesses and reports said they had fled the Tigrayan capital.

Billene Seyoum, the spokesperson for prime minister Abiy Ahmed, did not comment on the Tigrayan troops taking over Mekelle.

But she confirmed that the Ethiopian government on Monday had declared “an unconditional humanitarian ceasefire”. International organisations have sounded the alarm that there are hundreds of thousands living in famine-like conditions.

António Guterres, UN secretary-general, said on Monday night after speaking to Abiy that he was hopeful “that an effective cessation of hostilities will take place in Tigray”.

In recent weeks, diplomatic and humanitarian sources said, forces loyal to the TPLF had retaken territory lost to Ethiopian federal forces.

Getachew Reda, a senior member of the TPLF, said on Sunday that “our forces are making more and more gains”, but his claims could not be verified. On Monday, he told Reuters via satellite phone that Mekelle “is under our control”.

Fighting started in Ethiopia’s northernmost region of Tigray in early November after Abiy sent troops to oust the TPLF.

Government officials argue that he was left with no option but to invade Tigray after troops loyal to the TPLF attacked the federal army’s northern command. Abiy had promised that the operation would swiftly restore law and order, but the conflict has become a protracted war that has sparked alleged massacres, fuelling a humanitarian crisis.

It has also drawn in troops from neighbouring Eritrea, whose capital Asmara has been hit by rebel rockets and whose strongman leader, Isaias Afwerki loathes the TPLF. The conflict has horrified an international community that in 2019 awarded Abiy the Nobel Peace Prize.

The government in Addis Ababa had branded the TPLF, long the dominant party in Tigray, a “criminal clique”. As the leading member of the four-party coalition that ran Ethiopia for almost three decades until 2018, when Abiy took office, the TPLF had played a decisive role in national politics.

Tigrayans have a long history of fighting Addis Ababa. They did so in the 1940s against Emperor Haile Selassie in the so-called Woyane rebellion. Forty years later, they led a successful guerrilla war against the Marxist Derg regime, coming to power in 1991 after marching into Addis Ababa.

The unilateral ceasefire declaration followed several important military gains by fighters loyal to the TPLF, risk consultancy Eurasia said in a note on Monday, adding that Tigrayan fighters were “likely to accede to a temporary deal given that it provides both an opportunity to consolidate the wave of local support and to fortify its position in and around Mekelle”.

It warned, however, that “though this opens the door to dialogue toward a more comprehensive settlement on the conflict, discussions are likely to be difficult and protracted”.