By Steve Gutterman

MOSCOW (Reuters) - An ethnic Russian woman, who was both wife and widow of Islamist militants, was named on Wednesday as the suicide bomber who killed a moderate Muslim cleric in the North Caucasus just as President Vladimir Putin was pleading for national unity.

Tuesday's assassination of Said Atsayev, 74, a prominent Sufi sheikh in the troubled province of Dagestan who had spoken out against violent Islam, heightened tensions which Putin, visiting another Muslim region, had been trying to calm.

Police said Aminat Kurbanova had posed as a pilgrim to the cleric's home and detonated an explosive belt packed with nails and ball bearings, killing Atsayev, herself and six others, including an 11-year-old boy visiting with his parents.

A security source said the woman, aged either 29 or 30, was born with the ethnic Russian family name Saprykina but converted to Islam and was married to an Islamist militant. Two previous husbands, also militants, had been killed, the source added.

Suicide missions by wives of fallen fighters, dubbed "Black Widows", has been a feature of guerrilla groups from Chechnya and neighboring Muslim regions in the past decade.

The bombing came as Putin was visiting Tatarstan, a Muslim region in central Russia. While there, he made a rousing call for religious and ethnic concord to counter extremism that has raised new concerns about the integrity of a vast nation which is home to a wide mix of faiths and cultures.

"In Dagestan, sheikh and peace blown up," read the front-page headline on Moscow newspaper Kommersant, which said 80,000 people attended the cleric's funeral after dark in his village. Widely respected, Atsayev had helped broker a pact this year to reconcile some radical Salafist Muslims with the mainstream.

His death increased tension in Dagestan in particular, prompting an official day of mourning locally, though attacks occur almost daily. They are linked to an Islamist insurgency across the North Caucasus following two post-Soviet wars pitting the Kremlin against separatists in neighboring Chechnya.

The province also saw a bloody incident on Tuesday, in which a border guard killed seven fellow soldiers at a frontier post before being shot dead. Some Russian media suggested the killer may have been recruited by Islamist militants, but officials said the matter was still under investigation.

PUTIN PLEDGE

Though some ethnic Russians have fought alongside the Islamists in the North Caucasus, the killing of Atsayev, also known as Sheikh Said Afandi al-Chirkavi, appeared to be the first such case of an ethnic Russian suicide bomber.

Insurgents in Dagestan frequently attack government and security officials and have also increasingly targeted traditional mainstream Muslim leaders who are backed by the authorities. Atsayev was among the most prominent of these.

Putin owed some of his initial popularity to his launching, when prime minister, of a second war against Chechen separatists in 1999. He then swiftly succeeded Boris Yeltsin as president.

Now, the 59-year-old leader, who started another six-year presidential term in May, is eager to prevent the militant Islam that has flourished during the insurgency in the Caucasus from gaining ground in other regions with large Muslim populations.

"We will not allow anyone to tear our country apart by exploiting ethnic and religious differences," Putin said on Tuesday in Tatarstan, a long-peaceful region with substantial oil reserves, where the senior officially backed Muslim cleric was wounded last month and one of his deputies killed.

(Editing by Alastair Macdonald)