• The rebel Tigray Defence Force (TDF) said it had seized several towns, where witnesses have told the BBC they have seen its fighters patrolling.
• Following the fighting, some five million people in Tigray are in need of food aid and more than 350,000 are living in famine conditions.
Heavy fighting has broken out in several areas in Ethiopia's northern region of Tigray between rebels and federal troops, reports say.
The rebel Tigray Defence Force (TDF) said it had seized several towns, where witnesses have told the BBC they have seen its fighters patrolling.
The federal Ethiopian army dismissed the claim, saying it was fake news.
This is the most serious fighting since November when the Ethiopian government declared victory in the conflict.
Thousands of people have been killed and millions displaced in the war that started almost eight months ago.
Following the fighting, some five million people in Tigray are in need of food aid and more than 350,000 are living in famine conditions, according to a recent UN-backed estimate.
The reports of the fighting comes as ballots are being counted from Monday's general election. No voting took place in Tigray because of the security situation.
Rebel spokesman Gebre GebreTsadiq said that the TDF had launched the attack last week targeting several towns. He said the fighters had destroyed military vehicles and captured some soldiers.
Witnesses have told the BBC that rebel forces have entered the strategic town of Adigrat, which is just 45 km (30 miles) from the Eritrean border. If confirmed, this would be the most significant rebel advance since the war began.
TDF fighters have also been seen in several towns both to the north and south of the region's capital of Mekelle.
Army spokesman Colonel Getnet confirmed there was fighting but denied that any towns, army equipment or soldiers had been captured.
"While the Ethiopian government was busy with the national elections and the GERD [Grand Ethiopian Renaissance Dam] issues, the terrorist TPLF [a key part of the TDF], along with its young recruits, was actively involved in terrorist activities," he said.
He added that operations were underway to capture the rebel leaders.