Airstrikes and artillery fire hit northern Ethiopia's Tigray region overnight into Monday morning, aid sources said, almost two weeks into clashes between federal government and rebel forces that have raised fears of wider fighting.

Air attacks killed at least 17 people northwest of Tigray's capital Mekelle - which federal Ethiopian National Defence Force soldiers took over the weekend - according to a bulletin circulated among humanitarian groups.

The bulletin added that government forces exchanged artillery fire with fighters loyal to the Tigray People's Liberation Front - the region's dominant political force which is part of a rebel alliance aiming to overthrow Prime Minister Abiy Ahmed's government. 

Spokespeople for Abiy's office, the federal forces and the TPLF did not immediately respond to requests for comment on the reports, which Reuters could not confirm independently. 

Analysts say the fighting threatens to escalate into a wider conflict, with Ethiopia accusing Eritrea, Sudan and Egypt of supporting the armed opposition. All three countries have denied this. 

Last week, Ethiopia and Eritrea, which fought a border war from 1998-2000, severed diplomatic ties over the allegations.

WORST ESCALATION SINCE 2020-2022 CIVIL WAR

The fighting since September 23 is the worst escalation of violence since a 2020-2022 civil war in Tigray that resulted in hundreds of thousands of deaths.

In that war, federal forces captured Mekelle within a month before the ‌TPLF reclaimed it seven months later. 

On Monday morning, Ethiopia's military and Tigrayan forces exchanged heavy artillery fire close to the Tekeze River near the town of Mai Tsebri, according to the humanitarian bulletin. 

Overnight, airstrikes killed at least nine people in Adwa, which is about 100 km northwest of Mekelle, and eight in Sheraro in northwestern Tigray, the bulletin said.

It did not say who launched the airstrikes.

Another aid worker confirmed the strikes in Adwa, but did not have information about casualties.

In Mekelle, which Tigrayan forces left without a fight, daily life was returning to normal, with shops reopening and phone and internet access returning a week-long disruption, residents said.  

US WARNS ETHIOPIA AND ERITREA AGAINST CONFLICT 

Analysts are closely watching the Ethiopian military as it advances northward toward the Eritrean border. 

Eritrean forces fought in support of Ethiopia's federal government in Tigray during the 2020-2022 war following a 2018 normalisation of ties between the longtime foes that won Abiy the Nobel Peace Prize.

Relations soured afterward, in part over landlocked Ethiopia's public insistence that sea access is vital for its security and economy.

Eritrea says it fears Ethiopia will try to take its Assab port by force. Abiy has said Ethiopia intends to secure sea access through peaceful means. 

On Saturday, the US State Department urged Asmara and Addis Ababa to exercise restraint, warning that a conflict would jeopardise US collaboration with both countries.