Attacks in Iraq killed 13 people on Tuesday, four of them Shiite worshippers, while militants bombed a major oil pipeline, halting exports via Turkey, officials said.

Security forces have launched major operations against militants in recent weeks that are said to have resulted in scores of arrests, including 12 people detained on Tuesday on suspicion of planning a massive assault on Abu Ghraib prison outside Baghdad last month.

But they have yet to curb a surge in violence that has killed more than 3,400 people this year.

In the deadliest attack on Tuesday, a car bomb exploded after midday prayers at a Shiite place of worship in Al-Zahraa, south of the capital, killing four people and wounding 14.

Militants have carried out attacks on both Sunni and Shiite mosques this year, raising fears of a return to the all-out sectarian conflict that peaked in 2006 to 2007 and killed tens of thousands of people.

In the northern province of Kirkuk, a car bomb killed three police.

In Salaheddin province, also north of Baghdad, bombers killed a soldier, an anti-Qaeda militiaman and two civilians.

And in Nineveh province, in the north, gunmen shot dead a former soldier and a civilian.

Militants in the province also bombed the main oil pipeline from northern Iraq to Turkey, near the town of Albu Jahash.

The attack halted exports via the pipeline, a senior official from the North Oil Company said, adding that production was continuing but the oil was being stored.

Repairs to the pipeline, which runs from the northern oil hub of Kirkuk to the port of Ceyhan in Turkey and has been hit by dozens of attacks this year, are expected to take between one and three days, the official said.

The attacks came a day after bombs targeting a cafe, a football field and a market killed 28 people north of Baghdad.

Security forces arrested 12 people suspected of involvement in the July 21 assault on Abu Ghraib prison, two of them among the hundreds of inmates who escaped, an official in Iraq's Counter-Terrorism Service said.

The coordinated attacks on Abu Ghraib and Taji prisons, both claimed by an Al-Qaeda front group, killed more than 50 people and freed more than 500 inmates, officials said.

Authorities have hit back with a string of major security operations -- among the largest since US forces departed in December 2011.

The interior ministry announced on Monday that security forces had detained 82 suspected militants in Salaheddin and Diyala provinces, 56 of them at an alleged Al-Qaeda training camp.

Violence in Iraq has increased markedly this year, with analysts saying the upsurge is driven by anger among the Sunni Arab minority that the Shiite-led government has failed to address, despite months of protests.

Attacks have killed 3,421 people in Iraq since the beginning of 2013, according to figures compiled by AFP -- an average of more than 15 per day.