Ethiopian Prime Minister Abiy Ahmed has said that he is launching the "final phase" of the army's operation in the northern region of Tigray after weeks of fighting, BBC reported. He said the military would try not to harm civilians in the regional capital Mekelle - a city of 500,000 people - and urged residents to stay at home. The TPLF party, which controls Mekelle, has vowed to keep fighting. The UN warns of possible war crimes if the Ethiopian army attacks Mekelle. UN human rights chief Michele Bachelet told the BBC the city's inhabitants were in "deep peril". "We're really alarmed by the dangerous situation particularly because of the civilians trapped in Mekelle," she said, adding that there was "potential for serious violations of international humanitarian human rights law". The UN has also expressed concerns about the lack of access to the region for humanitarian workers. The Ethiopian authorities on Thursday said that "a humanitarian access route" overseen by the government will be opened up, adding that they were "committed to work with UN agencies... to protect civilians and those who need it". Mr Abiy's announcement on the military operation came after a deadline he gave for Tigray fighters to surrender expired on Wednesday. Hundreds of people have reportedly been killed and thousands have been forced from their homes as Ethiopian forces have seized various towns in Tigray from the TPLF. Details of the fighting are hard to confirm because all phone, mobile and internet communications with the Tigray region have been cut, BBC said.