RIYADH, Saudi Arabia — More than five years have passed since Saudi Arabia's King Abdullah last received Iraq's prime minister, Nouri al Maliki. The Saudi monarch views Maliki as untrustworthy and, even worse, "an Iranian agent."

Saudi Arabia doesn't allow direct flights between its capital, Riyadh, and Baghdad, and it doesn't permit direct trade between the two countries. The kingdom is building a fence along the closed 500-mile border.

This, too, is a legacy of the U.S. invasion of Iraq as U.S. troops complete their withdrawal: a bitter enmity between two close U.S. allies, with an underlay of sectarian animosity, that the United States cannot seem to ameliorate.

It is an irony, because the U.S. first sent troops to the region in part to protect Saudi Arabia in the wake of Saddam Hussein's 1990 invasion of Kuwait. Thirteen years later, however, when the U.S. invaded Iraq to topple Saddam, Saudi rulers were highly critical. And they have remained opposed to or offended by almost everything that has happened since.

Saudi Arabia refuses to set up an embassy in Baghdad, and while it has allowed Iraq to set up a mission in Riyadh, its officials receive Iraqi government officials only as private individuals.

The Saudis charge that Iraq has come under the sway of Saudi archrival Iran. But they themselves have also tried to affect Iraqi internal politics: they've thrown their support and funds behind Ayad Allawi, Maliki's main political rival, who's blocked the appointment of top security officials in the Iraqi government.

"We're trying to contain them ... it's a sectarian government," said an adviser to the Saudi government who agreed to discuss the delicate Saudi-Iraqi relations anonymously because he wasn't authorized to speak to the media.

For its part, Iraq charges that insurgents are still infiltrating from Saudi Arabia. "There are a lot clerics and religious organizations that encourage and incite people to go to Iraq and fight in a so-called jihad," Labeed Abbawi, Iraq's deputy foreign minister, told McClatchy in Baghdad, skirting direct criticism of the Saudi government.

The enmity between Saudi Arabia and Iraq is just one of the many fissures in the Middle East that have widened in the almost nine years since the U.S. toppled Saddam. Now, the Arab Spring has exacerbated already existing sectarian tensions in the region at a time when the U.S. departure from Iraq leaves it with less capacity to act in the region to intervene if military conflict seems imminent.

The biggest fissure is the division between Sunni-led Saudi Arabia and Shiite-led Iran. The two countries have been at loggerheads for centuries, in large part over whose branch of Islam should lead the Muslim world. But the replacement of Saddam Hussein's Sunni-led regime with Maliki's Shiite-led government unsettled the playing field for the two countries.

The current revolts in Syria and Bahrain offer new venues for Iran and Saudi Arabia to battle for influence.

In Syria, the Saudis see the overthrow of Bashar Assad as a sectarian gain for Sunni Islam, ending the domination of the Sunni majority by Assad's Alawites, a sect related to Shiism.

For Shiite Iran, the overthrow of Assad is an "existential concern," say U.S. officials. Iran uses Syria as its conduit to Lebanon, and if cornered, Tehran might lash out, analysts said.