European Union foreign policy chief Catherine Ashton on Thursday sounded a strong note of alarm over the escalation of violence in recent days in the capital of the Central African Republic.
"I am alarmed by the deteriorating situation in Bangui and the escalation of violence there since March 22," she said.
Attacks against foreign peacekeepers seeking to stem inter-religious strife were "unacceptable", she said in a statement.
"The combined action of the African force MISCA and the French operation Sangaris is essential to contain the tension and to restore public order as soon as possible," she added.
The head of the MISCA force this week said members of the majority-Christian vigilante group known as "anti-balaka" had crossed a red line by attacking the international troops and would be treated as "enemies".
Around 20 people have died in clashes involving armed groups and foreign peacekeepers in Bangui since Saturday.
Some 8,000 foreign troops are working to disarm Central African rebel groups after a year of inter-religious violence.
Security is especially tight ahead of this week's first anniversary of the toppling of Francois Bozize by majority-Muslim Seleka rebels, which sparked the current unrest.
Ashton urged the international community to "act swiftly" to prevent further bloodshed and said the EU would take part in efforts to restore stability and security in Bangui and the rest of the country.
The so-called "anti-balaka" militias were formed in response to killing and pillaging by Seleka rebels who went rogue after last year's coup.
Thousands have been killed and around a quarter of the country's 4.6 million people displaced. Muslim residents of Bangui have been besieged for weeks by the anti-balaka as well as by looters.
Speaking at an EU summit in Brussels last week, French President Francois Hollande appealed to his European partners to quickly provide enough troops for a delayed EU peacekeeping mission to the Central African Republic.
"Five hundred personnel are needed. There are currently only 375, so more effort must be made," Hollande said.