The capital of Ethiopia's northern Tigray region has come under heavy shelling from government forces, aid workers and regional officials say.
The centre of Mekelle is being hit "with heavy weaponry and artillery," the ruling regional party said.
The Ethiopian army has been locked in conflict with the Tigray People's Liberation Front (TPLF) for weeks.
It says it hopes to seize the city from the TPLF within days, but will avoid harming its 500,000 civilians.
Hundreds of people have reportedly been killed, and thousands have been forced from their homes as Ethiopian forces have seized towns.
The Ethiopian military earlier said it had captured the town of Wikro, north of Mekelle, along with several other towns in the region.
Details of the fighting are hard to confirm because all phone, mobile and internet communications with the Tigray region have been cut.
What's the latest from the Tigrayan capital?
TPLF leader Debretsion Gebremichael told Reuters in a text message that Mekelle was under "heavy bombardment", and said that government forces had started an operation to capture it.
A separate TPLF statement, reported by AFP, urged "the international community to condemn the artillery and warplane attacks and massacres being committed".
It also accused the Eritrean government of involvement in the attack on Mekelle.
Aid agencies and diplomats have been told by residents there have been explosions in the north of the city.
The Ethiopian government has not commented on the latest development. It said on Friday it had advanced to within 12 miles (20km) of the city and was beginning the "final phase" of an offensive.
A spokeswoman for Prime Minister Abiy Ahmed said Ethiopian forces would not "bombard" civilian areas.
"The safety of Ethiopians in Mekelle and Tigray region continues as priority for the federal government," Billene Seyoum added.
In a meeting on Friday, Mr Abiy told African peace envoys that civilians would be protected.
However, there was no mention of potential peace talks and the envoys were not allowed to visit Tigray.
The TPLF party, which controls Mekelle, has vowed to keep fighting.
The UN had warned of possible war crimes if the Ethiopian army attacked Mekelle.
It has also expressed concerns about the lack of access for humanitarian workers.
The Ethiopian authorities said on Thursday that "a humanitarian access route" overseen by the government would be opened, adding they were "committed to work with UN agencies... to protect civilians and those who need it".
On Thursday, Ethiopian troops were deployed along Tigray's border with Sudan, preventing people fleeing the violence from leaving the country, according to refugees.
The BBC's Anne Soy, on the Sudanese side of the border, saw at least a dozen members of the Ethiopian military, leading to a marked decrease in the number of people crossing into Sudan.
Who are the TPLF?
The TPLF fighters, drawn mostly from a paramilitary unit and a well-drilled local militia, are thought to number about 250,000. Some analysts fear the situation could turn into a guerrilla conflict - with the TPLF continuing to mount attacks on government forces even if they take Mekelle.
Leader of the TPLF, Debretsion Gebremichael, has said Tigray forces are "ready to die in defence of our right to administer our region".
Reuters quoted a diplomatic source as saying the TPLF had "mobilised lots of people in Mekelle". The person added: "They are digging trenches and everyone has an AK-47 [rifle]."
Aid groups fear the conflict could trigger a humanitarian crisis and destabilise the Horn of Africa region.
Ethiopia's state-appointed Human Rights Commission has accused a Tigrayan youth group of being behind a massacre this month in which it says more than 600 non-Tigrayan civilians in the town of Mai-Kadra were killed. The TPLF denied involvement.
What is the fighting about?
The conflict is rooted in longstanding tension between Ethiopia's government and the TPLF, which was the dominant political force in the country until Mr Abiy came to power in 2018 and introduced far-reaching reforms.
When Mr Abiy postponed a national election because of coronavirus in June, relations further deteriorated.
The TPLF said the government's mandate to rule had expired, arguing that Mr Abiy had not been tested in a national election.
In September the party held its own election, which the government said was "illegal".
Then, on 4 November, Mr Abiy announced an operation against the TPLF, accusing it of attacking the army's northern command HQ in Mekelle.