The US withdrawal from Iraq will leave a power vacuum in the Gulf, analysts say, paving the way for Iran to increase its influence in this economic and politically strategic region, a concern echoed by America's Gulf allies.

"The US withdrawal from Iraq will no doubt create a power vacuum," said Abdulaziz Sager, chairman of the Gulf Research Centre, noting that the US presence in the region empowered Gulf Arab countries with "whom they share interests and are bound by security agreements."

"This (US) presence gave them (Gulf countries) a sense of stability and security due to America's military capabilities," said Sager, adding that the troop withdrawal will "strengthen Iran's military and intelligence influence," in Iraq.

After nine years in Iraq, the US will withdraw its last remaining troops by the end of 2011, a year that so far has seen unprecedented uprisings in the Arab world that have unseated three entrenched dictators in Tunisia, Egypt and Libya, but also fuelled the sectarian and religious divide in the region.

Gulf Arab countries are primarily concerned about what Iran might do in what is today a Shiite-led Iraq, as well as their intentions in the broader region, including Saudi Arabia and Bahrain where Sunni regimes have harshly quelled Shiite dissent in recent months.

According to Sager, Iran's increased clout may not pose a "direct threat to Iraq" but will rather be used by the Shiite nation to challenge its rivals in the primarily Sunni Gulf, and to compensate for the potential loss of its ally Syria which is experiencing a mass uprising.

Iraq shares a religious link with Iran, and both nations have expressed similar critical views of the mounting regional and international pressure on Syrian President Bashar al-Assad to resign.

"On the political front, the US exit will likely push Iraq to seek closer relations with Iran, particularly given the complete absence of Saudi and Turkish influence in the country," Sager said.

US government officials, who have been working to calm concerns in the region, have argued that Iran in fact has failed to make Iraq a "client state," and that the Islamic republic is growing increasingly isolated.

US National Security Adviser Tom Donilon said last month that the "regional balance of power is tipping against Iran," and that Iraq and Iran have "very different visions of their future."

Tensions between Iran and its Arab rivals have risen in recent months as the Sunni-ruled monarchies in Saudi Arabia and Bahrain continue to accuse Tehran of instigating unrest among their Shiite populations.

Earlier this year in Bahrain, Shiite-led protests calling for democratic reforms were brutally quashed, leaving dozens dead.

An independent investigation into the crackdown found no evidence of Iranian involvement, but the Bahraini government has insisted that Iran's propaganda played a role in the uprising.

In Saudi Arabia's Eastern Province where several Shiite anti-government protesters were killed last month, the interior ministry blamed gunmen with foreign agendas of infiltrating the protesters and opening fire on security forces.

There are also concerns that Iraqi forces are simply not ready to take control of the country, which has reeled from bloody sectarian conflict and crippling political instability since the US invasion in 2003.

"The Iraqi security forces have built capacity to deal with internal threats for the last eight years," Lieutenant General Robert Caslen, the chief of the Office of Security Cooperation-Iraq (OSC-I), said in a recent interview with AFP.

But they "have not yet built the capacity to deal with the external threats," said Caslen.

Iraq's military chief of staff, Babak Zebari, has also conceded that Iraq may not be able to fully defend its borders, saying it will be years before the military can "perform all its external defence duties."

Iran analyst and head of the Centre for Strategic Studies in Kuwait, Sami al-Faraj, argues that Iran's military capabilities do not compare with the Gulf Arab nations, which he says will be able to fend off any potential Iranian aggression.

"Iran's weapons are old and worn out," said Faraj, adding that Iraq and its Gulf neighbours would be able to withstand an Iranian attack.

Political analyst Sami al-Nisf, meanwhile, said the timing of the American withdrawal from Iraq was unfortunate and will only complicate the region's existing political and sectarian challenges.

"The Americans should have stayed (in Iraq) as they did in Germany and Japan so the country does not become totally under Tehran's control," Nisf said, adding that "unfortunately, we are moving in that direction."