Ethiopian government forces have launched an offensive to capture the regional capital Mekelle in the rebellious northern Tigray region, according to a diplomat and the leader of Tigrayan forces.
Tigray People's Liberation Front (TPLF) leader Debretsion Gebremichael said Mekelle was under "heavy bombardment".
The Ethiopian military is using artillery in the assault, he said.
The diplomat said explosions were reported in the north of the city, in the Hamidai area. A second diplomat also said the attack had begun.
Spokeswoman for the prime minister's office Billene Seyoum said Ethiopian forces did not have a mission to bombard its own city and people.
"Mekelle remains one of Ethiopia's key cities and the efforts to bring to justice the criminal clique will not entail discriminatory 'bombardment' as alluded by TPLF and their propagandists," she said.
"The safety of Ethiopians in Mekelle and Tigray region continues as priority for the federal government".
Claims from all sides are difficult to verify since phone and internet links to the region have been down and access has been tightly controlled since fighting began three weeks ago.
The government gave the TPLF an ultimatum last Sunday to lay down arms or face an assault on Mekelle, a city of 500,000 people. The ultimatum expired on Wednesday.
Ethiopian Prime Minister Abiy Ahmed told African peace envoys on Friday that his government will protect civilians in Tigray.
But Abiy has said he regards the conflict as an internal matter and his government has so far rebuffed attempts at mediation.
The Ethiopian military has been fighting forces in the northern region of Tigray, which borders the nations of Eritrea and Sudan, since November 4.
Abiy accuses Tigrayan leaders of starting the war by attacking federal troops at a base in Tigray.
The TPLF says the attack was a pre-emptive strike.
Thousands of people are believed to have died and around 43,000 refugees have fled to Sudan.
Australian Associated Press