By Claudia Parsons

UNITED NATIONS, Sept 22 (Reuters) - The United States will press Iraq's neighbors and world powers on Saturday to implement U.N. pledges to do more in Iraq at a high-level meeting called by U.N. Secretary General Ban Ki-Moon.

The meeting on the sidelines of the U.N. General Assembly brings together U.S. Secretary of State Condoleezza Rice with her Iranian counterpart Manouchehr Mottaki at a time of tension over Iran's nuclear ambitions. Washington also accuses Iran of backing militants in Iraq.

At U.S. and British urging, the U.N. Security Council last month voted to assign the United Nations an expanded political role in Iraq, including promoting reconciliation between rival factions and dialogue with neighboring countries.

But many U.N. officials are deeply concerned about sending more staff to Iraq, remembering a bomb that destroyed its office in Baghdad in August 2003 and killed 22 people, including mission chief Sergio Vieira de Mello.

The U.N. Staff Union wants Ban not to deploy more people in Iraq and withdraw those there now.

Saturday's meeting, co-chaired by Iraqi Prime Minister Nuri al-Maliki, will bring together members of the Security Council, Iraq's neighboring countries, members of the Group of Eight (G-8) industrialized nations and representatives of concerned regional and international organizations.

"Secretary Rice will urge Iraq's international partners to follow through on their pledges on financial and debt relief to Iraq," said Kristen Silverberg, U.S. assistant secretary for International Organization Affairs.

Maliki met Ban earlier on Saturday to discuss how to expand the U.N. role in Iraq, as well as security issues.

'SITUATION HAS IMPROVED'

"The security situation has improved a lot in Baghdad," he said. "We are going to be able to provide security to the UN in a way that will allow it to perform its role in an effective manner," he told reporters after meeting Ban.

U.S. President George W. Bush boosted American troop levels this year to try to stabilize Baghdad and create a climate for political reconciliation between Iraq's Shi'ite and Sunni populations. But the Iraqi government has failed to meet several benchmarks for political reconciliation.

Bush recently backed a recommendation by the U.S. commander in Iraq, Gen. David Petraeus, to withdraw 20,000 troops by next July from 169,000 now in Iraq.

U.S. and British officials have denied that their aim is to unload Iraq's political problems on the United Nations, then pull their forces out. But they want the U.N. to take a shot at peacemaking, especially in recruiting help from neighboring nations.

Ban has already initiated a compact for Iraq that sets benchmarks for Baghdad in exchange for debt forgiveness.

Previous multilateral meetings on Iraq have provided rare opportunities for U.S. officials to meet representatives of Iran and Syria, both nations that Washington accuses of meddling in Iraq by supporting militant groups.

In May, Rice exchanged greetings with Mottaki in Sharm El-Sheikh, in Egypt, during a conference on debt relief and other measures to support Baghdad. Saturday's meeting will address whether countries have met pledges made then.

Maliki said there was no direct meeting scheduled between Rice and Mottaki in New York but "if circumstances allowed, we will work to make this meeting happen."

Western diplomats acknowledge Shi'ite Muslim Iran is an influential force on Iraq, both as a neighbor and because of its links with elements in the Shi'ite-led Iraqi government.

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