BAGHDAD (Reuters) - Iraqi security forces raided the office of a provincial governor and arrested the son of a leading Sunni Arab politician in separate incidents on Tuesday that could stoke sectarian and political tension.

In the early morning, an unidentified Iraqi security forces unit raided the office of the governor of Diyala province north of Baghdad, killing the governor's secretary and provoking clashes in which four people were wounded.

Separately, the head of the country's biggest Sunni Arab political bloc said Iraqi forces had arrested his son at their house in Baghdad and accused him of terrorism.

Adnan al-Dulaimi, whose Sunni Arab Accordance Front bloc rejoined the Shi'ite-led government last month after a boycott of nearly a year, said Iraqi army troops arrested his son Muthana outside their house while U.S. forces looked on.

"I asked them why they are taking him. They said they are preparing charges of terrorism, sectarian killings and displacing people," he told Reuters by telephone.

"My son is a humble man. He does not deal with political things. He only owns a shop selling spare car parts."

Another son of Dulaimi's has been held since last year along with many of the politician's bodyguards who were accused of links to a car bomb ring. Dulaimi has always denied the charges, which were a significant source of sectarian friction.

RAID HIGHLIGHTS FRICTION

The separate pre-dawn raid in Diyala province highlighted the tension in one of the country's most violent regions. Governor Raad Rasheed Mulla Jawad, who survived a bomb attack last week, said a counter-terrorism unit stormed his office in the provincial capital Baquba at 2:30 a.m. (1230 GMT).

They killed the governor's secretary, Abbas Ali Hmoud, and arrested senior provincial council member Hussein al-Zubaidy and the dean of the local university. Four policemen were wounded in clashes that followed between the raiding party and other units.

The provincial council announced it was suspending all work until it received an explanation for the assault. The governor called for a three-day period of mourning for his secretary.

"The body of the martyr will stay in the building until the killers are captured," the governor told Reuters. He was not present in the building during the raid.

Authorities offered no official account of who was behind the raid or its purpose. The men who conducted it arrived in military vehicles and wore uniforms but were not identifiable.

The U.S. military said it was not alerted in advance and could offer no further explanation.

Diyala, an ethnically and religiously mixed province in a fertile area north of Baghdad, has remained one of the most violent parts of Iraq even as the rest of the country has become safer over the past year.

U.S.-backed Iraqi forces launched a crackdown last month on Sunni Arab militants believed to be hiding out in its lush palm groves. The Americans and their Iraqi allies have also battled militias that they say have infiltrated the police.

Diyala's latest tensions appear linked to the firing of a police chief by the provincial council, days before last week's failed bomb attack on the governor. The issue has led to street protests.

(Additional reporting by Aws Qusay and Ahmed Rasheed, editing by Mark Trevelyan)