A small bomb exploded outside a synagogue in a central Siberian city amid a security alert in Russia over possible violence by right-wing skinhead groups. Neo-Nazis have also threatened to attack foreigners.

Police in the city of Krasnoyarsk said they're searching for two teenagers who may have placed the bomb outside the synagogue on Friday.

The bomb caused no damage and no one was injured. But the incident comes as police are out in force all across Russia in case of skinhead violence.

In Moscow, several outdoor markets run mostly by darker-skinned people from southern Russia closed down on Saturday, which is the anniversary of Adolf Hitler's birthday when neo-Nazi groups are especially active.

Last week various foreign embassies received an anonymous e-mail message warning of a "skinhead war" against foreigners as a way to mark the Nazi leader's birthday.

The warning took on new urgency when an Afghan man was beaten to death in a skinhead attack in central Moscow. The man worked as an interpreter for the Afghan embassy and leaves behind his Russian wife and four children.

Last October three people died during an attack by about 300 skinheads on an outdoor Moscow market, and a diplomat from Kenya was badly beaten in another recent attack.

Officials at the embassies of the United States, Japan and various African countries have issued warnings telling their citizens to take extra precautions this weekend.

About 10,000 Russians, mostly young men, are believed to be members of skinhead groups.

Last week President Vladimir Putin referred to the threat of extremist violence in his annual state of the nation speech.

Mr. Putin said extremism was a serious threat to the stability and security of Russia, and called for tough new laws in order to deal with the problem.

The Justice Ministry said it has drafted a law with tough measures against individual attackers as well as political groups which inspire them.