Five ethnic Chechens from Russia have been arrested in southern France, including one with a cache of explosives, a local mayor said as four other men appeared at a court in Paris, the first to face charges in the Paris terror attacks.

France has been on high alert since three days of terror left 20 people dead in Paris, including the three gunmen, earlier this month.

The report of the arrests came as France's prime minister urged his nation to do some soul-searching about the country's deep ethnic divisions and declared that fighting hatred, anti-Semitism and racism was a top priority, especially in France's impoverished housing projects, home to many immigrant communities.

Beziers mayor Robert Menard confirmed the five arrests in Beziers and on the outskirts of Montpellier. He said the man arrested in Beziers had been a resident "for some time".

Midi Libre, the local paper, reported that an explosives cache was found in Beziers near a stadium but prosecutor Yvon Calvet told Midi Libre it was not immediately clear whether a terror attack was planned. Prosecutors planned to hold a news conference later today in Beziers.

The Paris prosecutor's office said the four men in court were suspected of providing logistical support to Amedy Coulibaly, one of the terrorists killed by police, and requested the men be detained longer on weapons and terrorism charges.

Coulibaly shot a policewoman to death on the outskirts of Paris and then killed four hostages inside a kosher supermarket before being shot dead by police.

It is not clear whether the four suspects in court, all in their 20s, were involved in plotting the attacks or even aware of Coulibaly's plans.

The Paris prosecutor's office said five others arrested in the investigation were released without charge.

No-one has been charged with direct involvement in the January 7-9 Paris terror attacks. Coulibaly claimed allegiance to the Islamic State group while the two brothers who attacked the Charlie Hebdo satirical weekly said they were backed by al Qaida in Yemen.

Also today, French prime minister Manuel Valls told journalists that the attacks should force France to look at the "apartheid" within. The conservative Socialist whose hard line on Islamic extremism has won many fans said he was not making excuses for crime or terrorism, "but we also have to look at the reality of our country".

Mr Valls said memories have dimmed of the three weeks of riots by disaffected youths in 2005 that shook France.

"And yet, the stigmas remain ... a territorial, social and ethnic apartheid that has imposed itself on our country," he said. "The social misery is compounded by the daily discriminations, because someone does not have the right name, the right colour of skin, or because she is a woman."

In response to the 2005 riots, the French government spent hundreds of millions of euros to improve conditions in its rundown suburbs, with little success. Unemployment among young people in the housing projects is well above the national average and state buildings are often targeted for vandalism and arson.

"The fight against hatred, anti-Semitism in all its forms, racism - these fights are absolutely urgent," Mr Valls said. Young people who refused to take part in a national minute of silence for the terror attack victims "are symptoms of something that is not going well".

In Bulgaria, a court agreed to extradite a Frenchman who knew one of the two Kouachi brothers who massacred 12 people at Charlie Hebdo. Fritz-Joly Joachin told the Bulgarian court he was innocent and wanted to return to Paris to clear his name.

In Athens, an Algerian man suspected of jihadi terrorist links in Belgium appeared before a Greek prosecutor for an extradition hearing on being sent to Belgium. The suspect, whose name was not released, was detained Saturday in Athens, where he lives.

Belgium launched a large anti-terrorism sweep last week, during which two suspects were killed and one wounded, that netted several returnees from Islamic holy war in Syria.

Also today, New York mayor Bill de Blasio paid tribute to the victims of the terror attacks, placing wreaths of flowers at the Charlie Hebdo offices and near the kosher supermarket.