Iraqi authorities have tightened security in the Shi'ite holy city of Karbala, as thousands of pilgrims gather for the end of a festival already marred by deadly bombings.

Iraqi officials say they have deployed 30,000 security personnel in Karbala, including policewomen in charge of searching female pilgrims.

Millions of Shi'ites have visited the city over the past week for the ritual of Arbain, which culminates early Monday.  It marks the end of 40 days of mourning for 7th century Imam Hussein.

In another development, Iraqi officials say they have arrested a suspected al-Qaida militant near the western city of Fallujah in Anbar province.

Anbar's police chief told al-Arabiyah television that the Iraqi suspect, Sadi Nayif, carried out sectarian killings of Iraqi Shi'ites. Al-Qaida in Iraq is a Sunni militant group.

The Anbar police official says Nayif recently returned to Iraq from Syria.

Elsewhere, a bomb exploded Sunday in Baghdad's predominantly Shi'ite Sadr City district, killing one person and wounding 18 others.

In other violence, gunmen killed an Iraqi soldier and a civilian Sunday in separate attacks in the northern city of Mosul. The U.S. military also says an American soldier was killed Sunday in a bomb attack in southern Iraq.

Shi'ites heading to Karbala have come under attack several times in the past week, in bombings that killed at least 60 people. Some Sunni Islamist groups regard Shi'ites as heretics, and have attacked Iraqi Shi'ite pilgrims in the past.

Violence in Iraq has dropped sharply over the past year, but the latest attacks indicate the security situation remains fragile.

Meanwhile, Iraqi electoral officials say they have nullified provincial election results from 30 polling stations out of 6,500 across Iraq due to fraud. Iraq held provincial elections in 14 of its 18 provinces on January 31.

The officials said the fraud cases were not significant enough to require a new vote in any province. Final results from the elections are due to be announced in the coming days.

Preliminary results showed that allies of Iraqi Prime Minister Nouri al-Maliki came first in Baghdad and most provinces in southern Iraq. Polling day was largely free of violence, drawing international praise.

Some information for this report was provided by AFP, AP and Reuters.