Fears are mounting over the prospect of civil war after Prime Minister Abiy Ahmed sent federal troops and aircraft into the region with which Addis Ababa has been embroiled in a bitter feud.

Abiy said Friday that air strikes had already neutralised "rockets that can hit a range of 300 kilometres (186 miles) from where they are stationed in Mekele City and other places in the vicinity", state-affiliated Fana Broadcasting Corporate reported.

Three days after he announced a military operation against the state's ruling Tigray People's Liberation Front (TPLF) who he has described as a "criminal clique" bent on destabilising the country, lawmakers voted to abolish the state government.

The upper house of parliament "passed a decision to abolish the existing illegal Tigray regional assembly and executive, and for a caretaker administration to be formed," the state-owned Ethiopian Broadcasting Corporation (EBC) reported.

The house of federation's decision was based on a legal provision allowing federal intervention in a region deemed to have "violated the constitution and endangered the constitutional system."

"The caretaker administration will be mandated with conducting a constitutionally acceptable election and to implement decisions passed on by the federal government," the EBC said.

Despite growing international alarm, Abiy has vowed there will be more air strikes on Tigray, defending his move towards conflict as a "limited" operation necessary to restore law and order to the region.

"In order not to cause collateral damage, I call on civilians in cities to reduce mass gatherings," Abiy said Friday.

Abiy's statement indicated military operations were going well for federal forces, but a communications blackout in Tigray made this claim impossible to verify.

A UN source told AFP that an internal security report said Tigrayan forces held Ethiopia's Northern Command headquarters in Mekele.

- Long-simmering feud -

The key base is one of the most heavily armoured in the country, a legacy of Ethiopia's war with neighbouring Eritrea which borders the Tigray region.

The Tigray People's Liberation Front (TPLF) dominated politics in Ethiopia for nearly three decades before Abiy came to power in 2018 on the back of anti-government protests.

Tigrayans make up only about six percent of a population of more than 110 million people.

Their state is one of 10 ethnic-based federal regions that make up the country.

Under Abiy, Tigray's leaders have complained of being unfairly targeted in corruption prosecutions, removed from top positions and broadly scapegoated for the country's woes.

The feud became more intense after Tigray held its own elections in September, defying Abiy's government which had decided to postpone national polls due to the coronavirus pandemic.

Addis Ababa ruled the Tigray government was unlawful and in return Tigray said it no longer recognised Abiy's administration.

The federal government then slashed funding to the region, which the TPLF said was "tantamount to an act of war".

- Conflict would be 'devastating' -

As rhetoric heated up, Abiy, the winner of last year's Nobel Peace Prize, announced Wednesday he had ordered military operations in Tigray in response to an "attack" on a federal military camp by the TPLF.

The TPLF denies the attack occurred and accuses Abiy of concocting the story to justify deploying the military against it.

Diplomats and humanitarian sources say fighting has unfolded in the west, including along the border between Tigray and the Amhara region to the south.

Some 25 combatants -- the majority of them federal forces -- have been treated at a hospital in the Amhara town of Abdurafi, and five deaths have been reported there, a humanitarian source said Friday.

An additional 50 fighters have been treated in the town of Dansha, while a further 30 have been transferred to a hospital in Gondar, roughly 180 kilometres south of the Tigray border, the source said.

Unless the fighting stops soon, conflict in Tigray "will be devastating not just for the country but for the entire Horn of Africa," International Crisis Group said in a statement this week.

Given Tigray's considerable military capabilities and its estimated 250,000 troops, war could be "lengthy and bloody", it said.