The arrest of two prominent professors in Ethiopia's capital has given new focus to allegations that the authorities are engaged in a ruthless crackdown against ethnic Tigrayan civilians, as the country plunges deeper into a conflict that began over a year ago in the northern state of Tigray.

Professors Assefa Fissiha and Mehari Redeai both teach law at Addis Ababa University.

Although there is been no official confirmation of their arrests, multiple sources have told the BBC they were detained by the security forces for allegedly breaching the terms of Ethiopia's state of emergency, imposed as Tigrayan forces advanced towards the capital earlier this month.

Speaking by phone, family friends who confirmed the Tigrayan professors' arrests sounded fearful, anxious to remain anonymous in case they too faced detention in a country where, according to local and international human rights groups, at least 1,000 Tigrayans, and possibly many times that number, have been arrested in recent days.

Other Tigrayans, again speaking on condition of anonymity, have described a nervous mood in the capital, with people seeking to hide their accents and identities, or being forced out of jobs because of their ethnicity.

Among those allegedly affected are doctors, artists and policemen.

"I never imagined seeing such harassment in the city of my birth," one man told the BBC, saying he had watched 10 armed police officers taking his elderly father away from his home more than a week ago.

Ethiopian officials have defended the crackdown.

The head of the Ethiopian government communication office, Legesse Tullu, told local media that the arrests made under the state of emergency were not based on the ethnic identities of individuals.

Individuals who are suspected of supporting the Tigray People's Liberation Front (TPLF) and its ally the Oromo Liberation Army, which are designated as terrorist groups, have been arrested, he said.

He also added that there are more than 500,000 Tigrayans in the capital but the number of people arrested is not more than 1,000.

Prime Minister Abiy Ahmed has warned of a concerted attempt to twist the facts about the crackdown and the situation in Ethiopia more generally, tweeting about a "sinister" disinformation war against the nation and by urging Ethiopians to "reverse the distorted narrative".

But international criticism appears to be growing.

The UN's High Commissioner for Human Rights has accused the Ethiopian police of abusing the "excessively wide" terms of the state of emergency to round up Tigrayan civilians in Addis Ababa simply "on the suspicion" that they support the Tigrayan rebels.

Ethiopia's own Human Rights Commission has expressed similar "grave concern" about the detention of "thousands of people" in the capital and the difficulty of gathering information about the treatment of those in detention.

The commission said many arrests in Addis and elsewhere were the result of "tips" from other members of the community.

Driving this increasingly tense situation is the military offensive by Tigrayan rebels, who - together with a loose coalition of other rebel forces - are seeking to advance on the capital, to cut off key supply routes and, ultimately, to force the prime minister to step down.

The Tigrayans say Mr Abiy has forfeited the right to govern by waging a brutal military campaign in Tigray and by enforcing a de facto humanitarian blockade of the region that has left millions at risk of famine.

Ethiopia's government - which recently won a landslide victory in national elections - says the TPLF has attacked neighbouring regions and now threatens to tear the country itself apart.

International mediation efforts have, so far, achieved little and the prospects of a ceasefire appear slim at a time when both the TPLF and Ethiopian military, supported by regional security forces and ethnic militias, continue to claim the momentum on several fronts.

After a string of reported military setbacks, Mr Abiy imposed the state of emergency and urged civilians to take up arms to defend the nation.

But it remains unclear whether the TPLF - now fighting far from its home territory - will be able to seize, or surround the capital, or force a change of government.

The UK and US governments have urged their citizens to leave Ethiopia immediately, warning that the security situation in Addis itself could change "quickly and with little warning".

If Addis does come under growing threat from rebel forces, conditions for Tigrayan civilians in the city could well deteriorate.

Relatives of one Tigrayan doctor detained in the city said they had been unable to establish where he was being held.

There are reports that some families have been told they must pay bribes to secure the release of detainees.

"They are just misusing the decree and they are arresting people in order to collect money and profit themselves," one man told the BBC. Five police officers have been arrested for allegedly misusing their emergency powers.

"I have personally received news of several people known to me [being detained]," said the Norwegian academic and Ethiopia expert Kjetil Tronvoll.

"Local vigilante groups [have been observed] searching and detaining individuals suspected of being Tigrayans and turning them over to the authorities.

"The international community has been acting way too slow to arrest the deteriorating security situation in the country over the past year."