Medical charity Doctors Without Borders said Wednesday it was suspending work in some parts of Ethiopia's war-hit Tigray region after the "brutal murder" of three of its staff last month.
The decision is likely to fuel fears over access to life-saving aid in a region battered by eight months of conflict and where the United Nations says hundreds of thousands face famine.
At least 12 aid workers have been killed in Tigray since fighting broke out last November between the military and forces loyal to the region's former ruling party, the Tigray People's Liberation Front (TPLF).
Doctors Without Borders, known by its French acronym of MSF, announced on June 25 that one Spanish and two Ethiopian employees had been killed, although details of the attack including who was responsible are unknown.
"Almost two weeks since the murders of our colleagues, no one has claimed responsibility and the circumstances around their deaths remain unclear," MSF operations director Teresa Sancristoval said in a statement Wednesday.
"This is why we are requesting an immediate investigation by relevant parties to establish the facts of the incident that resulted in their deaths and to provide us with a detailed account of what happened and who was responsible."
Ethiopia said in June the MSF staff were killed in the area of Abi Adi, 50 kilometres (30 miles) from the regional capital Mekele, adding that the TPLF was operating there.
Tigrayan leaders have blamed the killings on government forces.
Sancristoval said MSF was taking the "extremely painful but necessary decision to suspend our activities in several areas of Tigray," including Abi Adi, Adigrat and Axum.
All three have seen heavy fighting, and Axum was the site of a massacre by Eritrean troops in late November that left scores of people dead.
Suspending activities "will have major medical and humanitarian repercussions for the population of central Tigray," the MSF warned.
The slain MSF workers were 35-year-old aid coordinator Maria Hernandez from Madrid; Yohannes Halefom Reda, a 31-year-old coordination assistant; and driver Tedros Gebremariam Gebremichael, also 31.
- 'Siege' fears -
The issue of humanitarian access has assumed fresh urgency since pro-TPLF fighters -- now renamed the Tigray Defence Forces (TDF) -- retook the regional capital Mekele last week.
Abiy's government declared a unilateral ceasefire hours later, but world leaders have warned of a possible "siege" of the region, with bridges going into Tigray destroyed and access restricted.
"There is an embargo on flights to Tigray, internet and telecommunications have been cut. Telecommunication outlets that are essential to humanitarian operators have been confiscated," EU crisis management commissioner Janez Lenarcic told the European Parliament Tuesday.
"It is not a ceasefire, it is a siege and starvation is used as a weapon of war."
MSF called on all parties to the conflict to ensure the safety of aid workers.
"Aid organisations, including MSF, have been repeatedly undermined by public statements casting unwarranted suspicion on their activities, thereby jeopardising the safety of their teams on the ground," it said.
Ethiopia has said accusations it wants to starve the Tigrayan population are "beyond the pale."
The government said Wednesday it had allowed humanitarian flights into the region, although a UN official told AFP none had taken off.
The United Nations has warned that roughly 350,000 people in Tigray are on the brink of famine, while the United States has said the figure is as high as 900,000.
- 'Bury our enemies' -
Tigrayan leaders have said they would only accept Abiy's call for a ceasefire if forces from Eritrea and Ethiopia's Amhara region -- who have been backing the Ethiopian army -- also withdrew from Tigray.
In a televised interview Tuesday, Tigray's pre-war president Debretsion Gebremichael said these forces were still in the region and hindering aid distribution.
"The humanitarian assistance isn't reaching the needy people in Tigray, rather it is supporting the invading forces," he said.
Analysts warn the next outbreak of fighting could centre on western Tigray, in territory annexed by Amhara forces who say it rightfully belongs to them.
In the interview, Debretsion indicated Tigrayan leaders were still very much on a war footing.
"We call upon the Tigrayan youth to continue with their volunteering to join our forces, until we bury our enemies," he said.