* Seized official is member of Sunni Islamist party
* Unclear if kidnapped by militia or detained by authorities
* Entire Shi'ite family beheaded in Taji (Adds Kurdish casualty, deaths of 12 volunteers)
By Raheem Salman
BAGHDAD, July 26 (Reuters) - Gunmen in army uniforms have seized a senior local official and prominent member of a Sunni Islamist party from his Baghdad home, police and security sources said on Saturday.
In a sign of the breakdown of security in and around Baghdad, 15 people, including an entire Shi'ite family, were found shot or beheaded, according to police and medical sources.
It was not clear if Riyadh al-Adhdah, who heads Baghdad's Provincial Council and belongs to the Sunni Islamist Iraqi Islamic Party, had been kidnapped by militiamen, who often wear military outfits, or detained by the authorities.
A police official speaking on condition of anonymity confirmed that men in army uniforms had taken Adhdah on Friday night.
Prime Minister Nuri al-Maliki's military spokesman was not immediately available for comment on the incident.
Security in Baghdad has been deteriorating as sectarian tensions deepen. The lightning advance of Sunni militants through northern Iraq last month has raised fears that Baghdad will be violently carved up along sectarian lines.
Iraq's last sectarian civil war reached its peak in 2006-2007 during the U.S. occupation.
Adhdah had previously faced terrorism charges but was not convicted due to a lack of evidence. Sunni politicians have long accused Maliki's security forces of targeting them on false terrorism-related charges in a witch-hunt.
Tensions between Sunnis and Shi'ites are increasingly at the forefront of the violence in Iraq, threatening to fragment the country, a major OPEC oil producer.
Gunmen beheaded a Shi'ite family of five in their home in the town of Taji just north of Baghdad, police and medics said.
Security forces, meanwhile, retrieved the bodies of six men in military trousers who had been handcuffed and shot in the head and chest in Taji, security sources said.
In eastern Baghdad, security forces found the corpses of four men who had been handcuffed, blindfolded and shot execution-style, security sources said.
Sunni insurgents led by the hardline Islamic State seized vast swathes of territory in the north last month, posing the biggest challenge to Maliki's Shi'ite-led government since U.S. forces withdrew in 2011.
Iranian-trained Shi'ite militias accused of sectarian killings have become a powerful force rivaling the battered Iraqi military in its ability to challenge the well-equipped and disciplined militants.
Twelve volunteers fighting alongside the army were killed in clashes with Sunni insurgents near the town of Baquba, 65 km (40 miles) northeast of Baghdad, a military source said.
Kurdish peshmerga forces are also taking on the Islamic State. The two sides have fought each other for control of Jalawla, 115 km (70 miles) northeast of the capital in recent weeks.
A senior Kurdish commander was killed in clashes on Saturday and another commander was kidnapped by insurgents in the town, security sources said.
Critics say Maliki, a Shi'ite, is a divisive figure whose alienation of Sunnis has fuelled sectarian hatred and played into the hands of insurgents.
The head of the Provincial Council's security committee told Reuters that Adhdah was seized along with four of his bodyguards from his home in the mostly Sunni district of Adhamiya by men in army uniforms driving SUVs.
Provincial councils are the top tier of local government in a system set up after the U.S. invasion in 2003. Members are elected.
Speaker of parliament Salim al-Jabouri, who is also a member of the Iraqi Islamic Party, said at a press conference after a meeting with Maliki on Saturday that the prime minister had a "big role" to play in Adhdah's case.
Maliki, who spoke to reporters after Jabouri, did not mention the incident.
(Writing by Maggie Fick; Editing by Michael Georgy, Raissa Kasolowsky and Lynne O'Donnell)