By Richard Lough and Sophie Louet

PARIS (Reuters) - French police have detained a Syrian asylum seeker in connection with the Normandy church attack, two sources said on Friday, as security services widened their investigation into the killing of an elderly priest at the altar by two would-be jihadists.

Three days after teenagers Adel Kermiche and Abdel-Malik Nabir Petitjean chanted in Arabic as they slit the throat of Father Jacques Hamel, investigators are probing their network of associates from the northern Normandy region to the alpine east.

A police source said the Syrian man was arrested near a refugee center in the rural Allier region of central France, where Petitjean lived for four years with his parents until 2012, according to French media.

A copy of the Syrian's passport was found at Kermiche's family home, the police source said.

A judicial source confirmed a Syrian was being held in custody. Two other individuals with suspected ties to the attackers are also being interrogated by police, the source said.

France is reeling from two strikes by assailants loyal to Islamic State in the space of 12 days. A Tunisian delivery man plowed his heavy goods truck through a crowd in Nice on Bastille Day killing 84 people.

The security record of President Francois Hollande and his Socialist government is under intense scrutiny following the revelation that Kermiche carried out his attack despite being under tight surveillance for two failed bids to reach Syria.

France had also been alerted by a foreign intelligence service that a suspected militant might be preparing an attack, with a nameless photo of Petitjean circulated among intelligence services.

The two men stormed a church service, forced the 85-year-old Roman Catholic priest to his knees at the altar and killed him. They were later shot and killed by police.

Unlike last year, when politicians mainly emphasized unity in the wake of militant attacks on the Charlie Hebdo satirical newspaper and on Paris entertainment venues, opposition politicians have strongly criticized Hollande's leadership in the wake of the recent attacks.

Nicolas Sarkozy, Hollande's predecessor and potential opponent in next year's presidential election, has called for stronger steps to track down and detain known Islamist sympathizers.

He has called for the detention or electronic tagging of all suspected Islamist militants, even if they have committed no offense -- an idea rejected by Prime Minister Manuel Valls.

"My government will not be the one that creates French-style Guantanamo centers," Valls told the Le Monde newspaper.

Valls said France had a strategy to defeat "Islamic totalitarianism" both in Iraq and Syria where it has launched hundreds of air strikes as part of a U.S.-led coalition, and at home. But he warned France would be targeted again.

"This war, which doesn't just involve France, will be long and we will be subjected to other attacks," he said.

Islamic State's affiliated news agency AMAQ released a new pre-recorded video late on Thursday purportedly showing Petitjean urging Muslims to destroy France.

Addressing Hollande and Valls directly, Petitjean says in the recording: "The times have changed. You will suffer what our brothers and sisters are suffering. We are going to destroy your country."

"Brothers go out with a knife, whatever is needed, attack them, kill them en masse," he says, calling on Muslims to attack allies of the international coalition fighting against Islamist militants in Syria.

(Reporting by Paris bureau; writing by Richard Lough; editing by Peter Graff)