By Stephen Kalin
BAGHDAD (Reuters) - Iraqi security forces investigating the abduction of 18 Turkish construction workers raided the Baghdad headquarters of a powerful Iranian-backed Shi'ite militia overnight, security sources and officials said on Friday.
Gunmen in military uniform had seized the Turks on Wednesday from a sports stadium they were building in northeastern Baghdad, in what Ankara said appeared to have been a targeted kidnapping.
The militia, Kataib Hezbollah, did not immediately respond to requests for comment.
Baghdad has struggled to rein in Shi'ite armed groups, many of which fought the U.S. occupation and are now seen as a critical deterrent against the militants of Islamic State, who have said they will march on the capital after seizing large swathes of the north and west in summer 2014.
The city has also seen a proliferation in recent years of well-armed criminal gangs carrying out contract killings, kidnappings and extortion.
Diplomats have said Turkey could suffer reprisals after abandoning months of reticence to launch air strikes against Islamic State in neighboring Syria and open its bases to a U.S.-led coalition fighting the Sunni Muslim militants.
Saad al-Hadithi, a spokesman for Iraqi Prime Minister Haider al-Abadi, said security forces had come under fire on Thursday night when they tried to raid a house on Palestine Street in Baghdad's eastern district of Mohandessen. Intelligence had indicated the presence there of a member of the group involved in the Turks' abduction, he said.
Hadithi would not confirm or deny that the suspect had been apprehended, and would not comment on his possible affiliation with Kataib or any other group.
A spokesman for the Hashid Shaabi, a government body overseeing armed groups fighting Islamic State including Kataib, denied the militia had any connection to the missing Turks.
Karim al-Nuri said a "routine search" had escalated into a quarrel that left one soldier dead and two militia members wounded.
"The friction started due to accusations that the Turkish workers were kidnapped by Kataib. Following the security forces' search, this allegation was proven wrong," Nuri said.
A security source said the army was searching the headquarters and surrounding buildings in the predominantly Shi'ite neighborhood, but had not yet found any trace of the Turkish hostages.
Turkish Prime Minister Ahmet Davutoglu spoke to his Iraqi counterpart and thanked him for his efforts, whilst vowing that Turkey would do everything necessary to ensure the hostages' release, sources in his office said.
Last year Turkey successfully negotiated the release of 46 of its citizens seized by Islamic State in the Iraqi city of Mosul, but only after they had been held captive for more than three months.
Separately, a Sunni tribal fighter, his wife and two children were shot and killed in the southern Baghdad suburb of Arab Jobour, police and medical sources said.
A bomb also exploded in al-Mashada, a predominately Sunni area of the capital's northern outskirts, killing two civilians and wounding five others, sources said.
There was no immediate claim of responsibility for either of the attacks.
(Additional reporing by Saif Hameed; Editing by Kevin Liffey)