Baghdad ? The slayings of two associates of Iraq’s top Shiite cleric raised fears Friday of a worsening Shiite power struggle in the country’s oil-rich south, prompting some clerics to go into hiding or abandon their robes and turbans for their own safety.

The two were killed late Thursday in separate shootings within 30 minutes in the southern cities of Basra and Diwaniyah. They joined at least four associates of Grand Ayatollah Ali al-Sistani who have been assassinated in the holy city of Najaf since June, including one stabbed to death about 30 to 40 yards from the house where the Iranian-born al-Sistani lives.

On Tuesday, an aide to al-Sistani was shot and seriously wounded by gunmen in Basra, 340 miles southeast of Baghdad. One of his guards was killed.

The attacks reflect the precarious security across much of Iraq and suggest that the competition for domination in the south is growing deeper and bloodier. That poses an additional threat to embattled Prime Minister Nouri al-Maliki, a Shiite whose 16-month-old government has been seriously undermined by a shrinking base of support.

The killings also have raised questions about the safety of Iraq’s four top Shiite clerics, particularly al-Sistani, who is known to have been the target of at least one assassination attempt since 2003. Like the three others, al-Sistani rarely leaves the house where he lives and works in Najaf’s old quarter, a labyrinth of narrow and dusty alleys.

Al-Sistani has played a key role in shaping the political future of Iraq following the 2003 overthrow of Saddam Hussein’s regime and wields considerable influence over Shiite politicians.

Iraq’s mainly Shiite south is a particularly coveted prize given its vast oil resources and the shrine cities of Najaf and Karbala, which attract millions of visitors every year and the cash gifts of wealthy Shiites abroad.

The region has been rocked in recent weeks by violence between rival militias linked to political parties. The main protagonists are the Mahdi Army militia loyal to radical cleric Muqtada al-Sadr and the Badr Brigade militia linked to the Supreme Islamic Iraqi Council, Iraq’s largest Shiite party.

The governors of two southern provinces were killed last month, and clashes between Badr and the Mahdi Army killed some 52 people during a major Shiite pilgrimage last month in Karbala.

Additionally, the recent withdrawal of British troops from central Basra to the nearby airport has threatened to allow Iraq’s second-largest city to become a free-for-all for rival Shiite factions.

As was the case with the earlier killings, no one claimed responsibility for the assassinations Thursday of Ahmed al-Barqaawi in Diwaniyah and Amjad al-Janabi in Basra, and no arrests were made.

Four other al-Sistani aides – Raheem al-Hisnawi, Abdullah Falak al-Basrawi, Kazim al-Bedeiri and Fadel al-Oukal – were slain in Najaf over a three-month period ending Aug. 2, according to security officials in the city.

The security officials, speaking on condition of anonymity because of the sensitivity of the subject, said several aides of al-Sistani have been threatened, forcing them to go into hiding in the city’s relatively safe old quarter or elsewhere in Iraq.

They said some had stopped wearing clerical robes or turbans when traveling outside Najaf.

The al-Hayat daily reported Thursday that some clerics were instructed to do so by their superiors in the Shiite religious hierarchy.