Clashes since Wednesday in Tigray and the neighbouring regions of Amhara and Afar have shattered a four-year-old peace deal that ended one of the century's deadliest wars.
The African Union estimated that conflict resulted in about 600,000 deaths from combat, famine and the collapse of healthcare.
The renewed conflict - in which the Tigray People's Liberation Front (TPLF) has joined forces with other Ethiopian armed groups aiming to overthrow the federal government - could further destabilise a region already unsettled by Sudan's civil war, frictions over Red Sea access and spillover from Middle East conflicts.
Calls and messages by Reuters reporters to numbers in Tigray on state-owned Ethio telecom's network did not connect on Friday while there was limited connectivity to private operator Safaricom's customers.
NetBlocks, a monitor that reports internet outages, confirmed the internet was disrupted in the area.
Four people with knowledge of the matter, including a diplomatic source, told Reuters that the TPLF had ordered both companies to shut down the networks.
One of the sources had said last week that reported TPLF restrictions on mobile communications may be part of a tactic to neutralise government forces' advantage in using battlefield drones, by preventing them from accessing the mobile network.
Drones provide a major battlefield advantage for the Ethiopian military against the TPLF, which is not known to have deployed them.
The military has large stocks of imported and domestically manufactured drones that it has been using to attack targets in Tigray since last month, according to the non-profit Armed Conflict Location & Event Data (ACLED) conflict monitor.
The TPLF did not respond to a request for comment.
A source close to the group, speaking on condition of anonymity, denied it was responsible and blamed the government.
The shutdown has sparked fears of a return to the dire conditions experienced by people in Tigray during the 2020-2022 war when the federal government severed communications, and access to food, cash and medicines was severely restricted, pushing hundreds of thousands of people into famine-like conditions.
This week's fighting prompted people to rush to shops to stock up on basic supplies and withdraw cash.
The Ethiopian National Defence Force's (ENDF) North East Command, which leads operations in northern Amhara and Afar, said in a statement on Friday that it had routed Tigrayan forces for the second time since Wednesday.
It said ENDF forces killed 436 enemy troops, wounded 220 and captured 44 after having claimed on Thursday to have killed 272 and wounded another 260.
A senior Tigrayan official said the ENDF's figures were not exaggerated.
Neither the official nor the ENDF provided casualty figures for federal troops.
The source close to the TPLF said federal troops had been put on the defensive by Tigrayan forces and their allies in Amhara and Afar.
He said clashes were taking place within kilometres of the Lalibela airport in Amhara, which is about 30km outside a holy town famed for its medieval rock-hewn churches.
Reuters could not independently confirm the battlefield reports.
The conflict began on Wednesday when Tigrayan forces and their allies seized three local airports and launched attacks against federal troops in Afar and Amhara.
The TPLF says it is engaged in a defensive war to counter aggression by Abiy's government.
The federal government has accused Eritrea of financially backing the TPLF and its allies, which Eritrean authorities deny.
Some Tigrayan residents, speaking on condition of anonymity, said forced recruitment efforts by the TPLF had intensified since Wednesday.
One resident of Maichew town told Reuters that TPLF fighters were still going house-to-house in search of youths, adding that he was hiding in his home and not going to work.
