"At 5am today (the TPLF) has attacked on the Eastern Front; from Bisober, Zobel and Tekulshe direction. By carrying out such measure, it has effectively broken the ceasefire," the government's communications service said in a statement.
The military command of the Tigrayan forces accused the government of violating the ceasefire, saying in a statement it believed the southern attack was a diversion and their forces expected a major attack from the west.
Tigray rebels accused Ethiopian government forces and allied militias of launching a "large-scale offensive" against southern Tigray on Wednesday.
There was no immediate response from the Ethiopian government and the claims by the Tigray People's Liberation Front (TPLF) could not be independently verified as the region is under a communications blackout.
"They launched the offensive early this morning around 5:00 am local time (0200 GMT). We are defending our positions," TPLF spokesman Getachew Reda told AFP in Nairobi in a brief message.
He said on Twitter that the "large-scale" offensive was launched "against our positions in the southern front" by the Ethiopian army and special forces as well as militias from neighboring Amhara.
The Ethiopian air force said Wednesday it had shot down a plane carrying weapons for the rebel Tigray People's Liberation Front (TPLF) that had encroached on the country's airspace via Sudan, state media reported.
"The airplane which violated our airspace from Sudan... and aimed to supply weapons to the terror group was shot down by our heroic air force," state media quoted armed forces Major General Tesfaye Ayalew as saying, without specifying the date of the incident.
The fighting is a major blow to hopes for peace talks between Prime Minister Abiy Ahmed's government and the Tigray People's Liberation Front (TPLF), the party that controls Tigray.
Both sides have blamed each other for the outbreak of fighting.
Ethiopian authorities on Wednesday accused Tigray rebels of launching a new offensive against federal forces in the country's north, five months after a truce paused the worst of the fighting.
"Disregarding the numerous peace options presented by the Ethiopian government, the armed wing of the terror group TPLF (Tigray People's Liberation Front), pushing with its recent provocations starting 5 am (0200 GMT) today committed an attack" in a region around southern Tigray, the Government Communication Service said in a statement.
Three residents reporting hearing heavy weapons since early morning. They also said there had been movement of Ethiopian soldiers, Amhara special forces and volunteer Fano militia in the past two days.
They said they did not know who had started the fighting. Reuters could not immediately get information on the movements of Tigrayan forces. Telephone connections inside Tigray have been down for more than a year.
Tigray has been without banking and communication services since the military pulled out at the end of June. Imports of fuel are restricted, limiting the distribution of aid.
The World Food Program (WFP) said in a report last week that since April 1 only 20% of the fuel needed to distribute humanitarian supplies had entered Tigray.
Almost 90% of people in the region need aid, the United Nations said, warning rates of malnutrition had "skyrocketed" and the situation was expected to get worse until October's harvest.
There is also shortages of some medical equipment and many drugs to treat common diseases like visceral leishmaniasis, say doctors at Ayder Referral Hospital, the biggest hospital in Tigray.
War erupted in Tigray in November 2020 and spilled into the neighboring regions of Afar and Amhara a year ago. Last November, Tigrayan forces marched towards Addis Ababa, but were driven back by a government offensive in November.
A ceasefire was announced in March after both sides fought to a bloody stalemate and the government declared a humanitarian truce, allowing badly needed food aid into the region.
Ethiopian government spokesman Legesse Tulu, military spokesman Colonel Getnet Adane, Amhara regional spokesman Gizachew Muluneh and the prime minister's spokeswoman Billene Seyoum did not respond to requests for comment.
The United States on Wednesday urged Ethiopia's government and Tigrayan rebels to hold talks to end renewed fighting, saying a five-month truce had saved "countless lives."
"We are concerned by reports of renewed hostilities in Ethiopia and we call on the government of Ethiopia and the TPLF to redouble efforts to advance talks to achieve a durable ceasefire without preconditions," State Department spokesman Vedant Patel said, referring to the rebel Tigray People's Liberation Front.
This report was sourced from information furnished by Reuters and Agence France-Presse