Prime Minister Abiy Ahmed's government, which has been embroiled in a year-long war against Tigrayan forces, has promised to keep fighting despite calls for a ceasefire from African nations, Western states and the U.N. Security Council.
"Incidents of civil unrest and ethnic violence are occurring without warning. The situation may escalate further and may cause supply chain shortages, communications blackouts, and travel disruptions," the U.S. Embassy said on its website.
Government spokesperson Legesse Tulu and Abiy's spokesperson Billene Seyoum did not immediately respond to requests from Reuters for comment.
Municipal authorities in the capital ordered residents who own firearms to register their weapons this week, to bolster defences in case the city is attacked. Addis Ababa has registered more than 10,000 weapons, Yonas Zewde, a spokesperson for the city administration, told state broadcaster EBC on Saturday.
Abiy's government declared a national state of emergency on Tuesday, saying it was locked in an "existential war" with forces from the northern Tigray region and their allies.
Getu Argaw, police commissioner for the capital, told EBC it was "only a dream" for the TPLF to think it could capture the city. He said police had confiscated weapons and uniforms from people in the capital.
Tigray People's Liberation Front (TPLF) spokesperson Getachew Reda accused Abiy of using the state of emergency to arrest "thousands of Tigrayans and Oromos".
The government spokesperson and the federal police spokesperson Jeylan Abdi did not immediately respond to Reuters requests for comment on Getachew's remarks but police have denied that arrests in the capital are ethnically motivated.
"We are only arresting those who are directly or indirectly supporting the illegal terrorist group," police spokesperson Fasika Fante said on Thursday, a reference to the TPLF. "This includes moral, financial and propaganda support."
The TPLF unveiled an alliance with other factions on Friday aiming to remove Abiy from power, saying this would be done by force if needed.
The government condemned the move, saying Abiy had a mandate to rule based on a landslide election win in June. It urged international partners to help protect Ethiopia's democracy.
The conflict in the north of Ethiopia started a year ago when forces loyal to the TPLF seized military bases in the Tigray region. In response, Abiy sent troops, who initially drove the TPLF out of the regional capital but have faced a sharp reversal since June this year.
Reuters has not been able to confirm independently the extent of the TPLF advance. The TPLF and their allies told Reuters this week they were now in the town of Kemise in Amhara state, 325 km (200 miles) from the capital. The government accuses the group of exaggerating its territorial gains.
The conflict has killed thousands of people, forced more than 2 million more from their homes and left 400,000 people in Tigray facing famine.
Social media companies Facebook and Twitter have taken action to limit what they call violations of their policies by Ethiopian accounts, including removing a post by Abiy's official Facebook account.
Twitter said on Saturday it had temporarily disabled the Trends section of its service in Ethiopia, which showcases the most tweeted subjects, because of threats of physical harm.
"Inciting violence or dehumanizing people is against our rules ... Given the imminent threat of physical harm, we've also temporarily disabled Trends in Ethiopia," the company said.
(Reporting by Addis Ababa NewsroomWriting by Duncan MiririEditing by Peter Graff and Edmund Blair)
