World Bulletin / News Desk
Four people were killed when a bomb blast tore through a minibus in western Ethiopia late on Tuesday.
The official, speaking to Reuters on Wednesday, said nobody had claimed responsibility for the blast.
Addis Ababa put its security forces on heightened alert on Tuesday night after receiving strong evidence that Somalia's al Shabaab group was plotting assaults.
"The bomb exploded on Tuesday inside a minibus travelling in Segno Gebeya," government spokesman Shimeles Kemal said, referring to a region bordering Sudan.
"No one has claimed responsibility yet. The case is under investigation."
The warning came three weeks after officials said two Somali suicide bombers accidentally blew themselves up while preparing for an attack on football fans during Ethiopia's World Cup qualifying match against Nigeria.
Al Shabaab has warned Ethiopia of revenge attacks for deploying troops inside Somalia to fight the al Qaeda-linked groups, alongside African Union forces from Uganda, Burundi and Kenya.
The National Intelligence and Security Service also urged the public on Tuesday to inform police if they encountered "suspicious" activity, and urged hotel staff and private landlords to verify the identity of visitors.