Tigrayan forces have said they have Ethiopian government troops on the run around the regional capital Mekelle after taking full control of the city in a sharp reversal of eight months of conflict.
People in Mekelle, where communications were cut on Monday, said the incoming Tigrayan fighters were greeted with cheers.
There were similar scenes on video footage from the northern town of Shire, where residents said government-allied Eritrean forces had pulled out and Tigrayan forces had entered.
"We are 100% in control of Mekelle," Getachew Reda, spokesman for the Tigray People's Liberation Front, told the media.
There had been some fighting on the outskirts of the city, but that was now finished, he said, adding that he could not confirm the report from Shire.
"Our forces are still in hot pursuit to south, east, to continue until every square inch of territory is cleared from the enemy."
The ousted TPLF was reestablishing itself in Mekelle and people could walk about again in the streets, he said, declining to be drawn on whether Tigray would now seek independence after a bitter conflict that often targeted civilians.
The Ethiopian government declared a unilateral ceasefire on Monday but has not publicly addressed TPLF claims to have taken back Mekelle.
Fighting in Ethiopia's northern region has killed thousands of people, displaced two million and brought hundreds of thousands to the brink of famine.
The US said atrocities should end immediately and warned Ethiopia and Eritrea that it would be watching closely.
"We will not stand by in the face of the horrors in Tigray," said Robert Godec, acting assistant secretary of state for the State Department's Bureau of African Affairs.
House Foreign Affairs Committee Chairman Greg Meeks said that if the disaster in Tigray is not abated, "we could witness one of our closest and most powerful allies in Africa march toward civil war and, eventually, a state collapse."
UN Secretary General Antonio Guterres said he hoped a political solution would be possible. Diplomats said the UN Security Council would discuss Tigray this week.
Ethiopia is awaiting results of national and regional parliamentary elections held on 21 June. Voting was only held in three of the nation's ten regions due to insecurity and logistical problems.
No voting was held in Tigray where the TPLF, an ethnically based political party that dominated Ethiopia's national politics for nearly three decades, has been battling the central government since early November. It made major territorial gains in the past week.
The fighting has been punctuated by reports of brutal gang-rapes and mass killings of civilians. At least 12 aid workers have been killed.
At least 350,000 people are facing famine and five million others need immediate food aid, the United Nations has said -the worst global food crisis in a decade.
Last week, an Ethiopian military airstrike on a crowded market killed at least 64 people and wounded 180 other people.
Doctors said women and children were among the dead and wounded and that Ethiopian troops prevented ambulances from reaching the scene for more than a day. The military said all the victims were combatants.
Ireland, US, UK request UN Security Council meeting on Tigray
Ireland, the US, and the UK called for an emergency UN Security Council public meeting concerning the war-torn Tigray region of Ethiopia.
Diplomatic sources said the meeting could be held on Friday, although it is up to France, which holds the security council presidency in July, to set an exact date and time.
Since the war began in November, the West has yet to succeed in organising a public session on Tigray, with many African countries, China, Russia and other nations deeming the crisis an internal Ethiopian affair.
These countries could request a procedural vote on the merits of the meeting prior to it taking place, which could nullify it.
Permanent member states would not be able to veto the move, but a vote by nine out of the 15 member states would be able to ensure the meeting goes ahead.