The US and UN on Wednesday called on Ethiopia to investigate an air strike that killed dozens of people in a busy market in the village of Togoga, in northern Tigray.
US State Department spokesman Ned Price condemned the "reprehensible act" on Tuesday, which killed at least 51 people and wounded more than 100, reports said.
"We urge the Ethiopian authorities to ensure full and unhindered medical access to the victims immediately," Mr Price said.
"We also call for an urgent and independent investigation, as well as remedial action, to hold those responsible for this attack accountable."
Ramesh Rajasingham, the UN’s acting head of humanitarian affairs, said he was “deeply disturbed” by the attack and reports of soldiers blocking roads and holding up ambulances.
“I call on the Ethiopian authorities to carry out a prompt and effective investigation into this attack and subsequent acts depriving victims of medical treatment, and to prosecute perpetrators of serious violations of international humanitarian law,” Mr Rajasingham said.
The air strike came amid some of the fiercest fighting to hit Tigray since the conflict erupted in November, when Ethiopian forces backed by neighbouring Eritrea and allied militias entered the region to remove the ruling Tigray People's Liberation Front.
Wounded patients being treated at Ayder Hospital in the regional capital, Mekele, told healthcare workers that a plane had dropped a bomb on the Togoga market, AP reported.
The patients included a two-year-old with “abdominal trauma” and a six-year-old, a nurse said.
An ambulance carrying a wounded baby to Mekele, almost 60km away by road, was blocked for two hours.
The baby died on the way, the nurse said, speaking on condition of anonymity for fear of reprisals.
Hailu Kebede, foreign affairs head for the Salsay Woyane Tigray opposition party, who comes from Togoga, said that one fleeing witness counted more than 30 bodies in the remote village.
A convoy of ambulances trying to reach Togoga, about 25km west of Tigray’s main city Mekele, was turned back on Tuesday afternoon by soldiers near Tukul, health workers said.
More ambulances were turned back later in the day and on Wednesday morning, but one group of medical workers reached the site on Tuesday evening by a different route.
“Attacks directed against civilians and indiscriminate attacks are prohibited, and all parties must take constant care to spare civilians throughout military operations,” Mr Rajasingham said.
This month, humanitarian agencies said that 350,000 people in Tigray are facing famine.
Aid workers have said they have been repeatedly denied access to several parts of the region.