The Tigray People's Liberation Front has admitted firing rockets on towns in neighbouring Amhara state and warned it will fire more on Eritrea, which it accuses of joining the fight.
TPLF claimed it attacked Gondar and Bahir Dar city, partially damaging the airports there on Friday night.
Spokesperson Getachew Reda said the attacks were in retaliation to the airstrikes on Tigray.
"The missile strike targeted military bases in Bahir Dar and Gondar," Getachew told Tigray Television in a statement also posted on the TPLF social media accounts.
"In the next two or three days will continue missile attacks not only on targets in Amhara region but also inside Eritrea" the spokesperson said.
"Whether they lift from Asmara or Bahir Dar to attack Tigray ... we will commit retaliatory measures. We will undertake missile attacks on selected targets in addition to the airports. We will conduct missile attacks to foil military movements in Massawa and Asmara", he added.
Ethiopia denies the claim that Eritrean soldiers are involved in the fighting.
Getachew spoke just a day after he and several other TPLF leaders were indicted by the Ethiopian Federal Police for crimes related to violating the federal Constitution.
The police have listed him, as well as more than 90 other politicians, military chiefs and diplomats, including Tigray President Debretsion Gebremichael, as wanted for treason and other crimes.
The regional TV station was on Saturday evening airing 'breaking news' items, saying the Ethiopian Air Force had bombed Wolkayt Sugar factory in Tigray.
This came as the Ethiopian National Intelligence and Security Service (NISS) said in a statement to the media that 14 members of Al-Shabaab and ISIS terrorists, who were planning to carry out attacks in Addis Ababa and other parts of the country, had been arrested.
Al-Shabaab fighters may have plotted the attacks at this time as Addis Ababa is focused on the situation in Tigray, north of the country.
Prime Minister Abiy Ahmed launched a military crackdown on TPLF on November 4, accusing the group of being "fugitives of justice" and said it had attacked the northern command of the Ethiopian National Defense Force.
Addis Ababa has since imposed a state of emergency on Tigray and shelled parts of the region thought to host artillery for TPLF.
The admission by Tigray's leaders raised fears that the escalating conflict could spread.
The attacks -- and threats of more to come -- fuelled concern that a conflict Abiy vowed would be quick and contained could instead snowball and destabilise the broader Horn of Africa region.
Hundreds of people are reported to have been killed in the conflict so far, some in a gruesome massacre documented by Amnesty International.
The strikes have also seen thousands of people flee into neighbouring Sudan, causing a humanitarian crisis.
As of Friday evening, at least 21,000 Ethiopians had fled across the border into Sudan, according to Sudan's refugee agency.
Abiy, the winner of last year's Nobel Peace Prize, on Friday declared the TPLF was in the "throes of death", but the party has vowed to fight on.
A communications blackout in the region has made it difficult to assess competing claims about how the fighting is going.
The TPLF dominated Ethiopian politics for nearly three decades before Abiy took office in 2018 on the back of several years of anti-government protests.
Since then, the TPLF has complained of being sidelined and scapegoated for the country's woes.
The feud grew more bitter after Tigray went ahead with its own elections in September -- defying a nationwide ban on all polls imposed because of the coronavirus pandemic -- and tried to brand Abiy an illegitimate ruler.