European nations, the White House and UN Secretary General Ban Ki-moon called for an investigation into violence which sparked fears of widespread sectarian unrest and stressed the need to defend all faiths in Egypt.

"I am very concerned, very alarmed about the clashes," British Foreign Secretary William Hague said at talks with his 26 European Union counterparts, in Luxembourg.

"It is very important that the Egyptian authorities reaffirm the freedom of worship," he said after the clashes that broke out during a Coptic protest against a recent attack on a church in the southern city of Aswan.

"The ability to worship in peace is a vital component of any free and democratic society."

The White House said Obama was "deeply concerned" about the situation and said it believed that minorities including Copts -- must be respected along with the rights to religious freedom and peaceful protest.

Spokesman Jay Carney said the "tragic events" in Egypt should not stand in the way of "timely elections and a continued transition to democracy that is peaceful, just and inclusive."

Ban called on Egyptian military authorities to defend "all faiths" in the country after the clashes and asked Egyptians to "preserve the spirit of the historic changes" witnessed during the Arab Spring uprisings, a spokesman said.

"The secretary general is deeply saddened by the loss of life in Cairo last night," said UN deputy spokesman Eduardo del Buey.

Germany meanwhile said it was "very worried" about the situation in Egypt.

"We can only call on the Egyptian government to get to the bottom of these incidents as soon as possible and bring those responsible to justice," Chancellor Angela Merkel's spokesman Steffen Seibert told journalists.

"We encourage the Egyptian government to do everything in its power to foster an atmosphere of religious tolerance," he added, calling for respect for the rights of the Coptic Christian minority.

German Foreign Minister Guido Westerwelle called on all sides to exercise restraint and moderation, while his Italian counterpart Franco Frattini urged EU "condemnation of the very serious violence against Christians, the Egyptian Copts", who he said were fleeing in "exodus".

Noting "an escalation" in violence against the country's Christians, Frattini said: "We hope the response of the Egyptian authorities will be more energetic than under (former president Hosni) Mubarak, which was insufficient."

At least 40 people were arrested in central Cairo when a Copt demonstration degenerated into deadly clashes that also left more than 300 people wounded.

It was not immediately clear how many of those detained were Muslim or Christian. Egypt's ruling military council, which took power when Mubarak was ousted in a popular revolt in February, ordered a probe into the clashes.

The events in Egypt were "extremely worrying," said EU foreign policy chief Catherine Ashton.

Ashton urged Egypt to move towards elections "with a desire to see all people part of those elections and to protect the people, whoever they are, wherever they come from, whatever belief and faith they have".

"We need to see the words of the Egyptian authorities turn to action."

Deep political change such as Egypt's almost always triggered "difficult moments", said Portuguese Foreign Minister Paulo Porta.

Spanish Prime Minister Jose Luis Rodriguez Zapatero "deeply regrets" the clashes, his office said in a statement.

Copyright AFP (Agence France-Presse), 2011