Satellite images of Tigray, Ethiopia-Sudan border

Ethiopia's prime minister said Thursday the army had been ordered to move on the embattled Tigray regional capital after his 72-hour ultimatum ended for Tigray leaders to surrender, and he warned its half-million residents to stay indoors and disarm.

Taking to Twitter, Prime Minister Abiy Ahmed said the "Final Phase of the Rule of Law Operations Commences", which means tanks and other weaponry can close in on Mekele, whose residents had been warned of “no mercy” if they didn't move away from Tigray leaders in time.

In an official statement, Abiy asserted that thousands of militia and special forces surrendered during the 72-hour period that ended Wednesday evening and that Ethiopia would "take utmost care to protect civilians”..

The United Nations has reported people fleeing Mekele, but communications and transport links remain severed to Tigray, and it’s not clear how many people received the warnings in time.

Tigray regional leaders couldn't immediately be reached.

The announcement has caused international alarm as rights groups fear such wording could violate international law and put civilians in further danger.

The international community is pleading for immediate de-escalation, dialogue and humanitarian access as Ethiopian forces have fought their way through Tigray to Mekele.

But Abiy, last year's Nobel Peace Prize winner, however, has rejected international “interference".

(Image credit: Abiy Ahmed/ Instagram)