(Recasts with more details)

By Mariam Karouny

BAGHDAD, March 10 (Reuters) - Iraq's prime minister urged regional rivals on Saturday to stop playing out their differences in his country, addressing a meeting at which U.S. officials sat down with adversaries from Iran and Syria.

The conference in Baghdad aims to stop sectarian warfare in Iraq and prevent the conflict's spread throughout the region.

In his opening speech, Prime Minister Nuri al-Maliki said all those with a stake in the peace of the Middle East should stand firm against terrorism in Iraq.

"We call on all to take moral responsibility by adopting a strong and clear stance against terrorism in Iraq and cooperate in stamping out forces of terror," Maliki said, according to the text of his speech to the opening session.

Iraq called the meeting to rally regional support to stop sectarian violence that threatens to tear the country apart, killed tens of thousands and driven some 2 million abroad since a U.S.-led invasion four years ago toppled Saddam Hussein.

"Our meeting with you today in Baghdad demonstrates support for the Iraqi people and Iraq's national unity government in our effort to confront all forms of terrorism in the country," Maliki said.

Two explosions rocked the conference venue around lunchtime. A Reuters witness said they appeared to be mortars landing nearby.

Iraq plans to invite foreign ministers from regional countries and the G8 group of industrialised nations to a meeting in April, a government spokesman said later.

Besides finding ways to stop the bloodshed in Iraq, the one-day meeting was a rare opportunity for old foes the United States, Iran and Syria to sit at the same table.

There was no indication delegations from the three countries held any bilateral talks. Washington and Tehran are at loggerheads over Iran's nuclear programme.

The United States has also accused Iran and Syria of fomenting violence in Iraq and supplying weapons and support for militant groups, charges both countries deny.

Security officials in the region have also identified a steady stream of Sunni extremists from neighbouring countries such as Saudi Arabia and Syria.

"We ... demand that regional or international states refrain from interfering or influencing the Iraqi state of affairs through supporting a certain sect, ethnic group or party," Maliki said.

The Shi'ite prime minister said national reconciliation between Iraq's Shi'ite and Sunni factions was the only way to prevent his country from sliding into all-out civil war and stop the conflict spreading beyond Iraq borders.

"The reconciliation initiative is the ship that will save us," Maliki added. "Iraq with its strategic location, must be seen as a key factor and an important player in the regional and international field."

The conference brings together mid-level officials from Iraq's neighbours, the permanent U.N. Security Council members -- the United States, Russia, China, Britain and France -- and Arab countries. There will be 16 delegations in all.

SPOTLIGHT ON IRAN-U.S.

"We call for peaceful dialogue to settle disputes, including the international, regional, inter-regional issues," he said, adding that Iraq was ready to play a positive role in finding a solution to such disputes," Maliki said.

In his speech, Maliki also demanded to see a "unified regional and international stand in support of the Iraqi people ... with no distinction along sectarian lines."

"We also wish to have our neighbours' support for confronting terrorism and look forward to more active international cooperation in reconstruction and development."

After Maliki's speech the delegates met behind closed doors for sessions expected to last most of the day.

The Iranian delegation arrived in the conference hall and took their seats without making any direct contact with U.S. delegates shortly before the meeting officially opened.

President George W. Bush said the U.S. message to Damascus and Tehran would be clear at Saturday's conference -- to help Iraq's "young democracy."

The United States would "defend ourselves and the people in Iraq from weapons shipped in to cause harm", he added.

Washington has led international efforts to impose tougher sanctions on Iran over its refusal to stop enriching uranium, which could be used for nuclear weapons. Tehran says its nuclear programme is peaceful.

Washington, which has no diplomatic relations with Iran, has had contacts with Iranian officials in group settings, including as recently as September, but has resisted bilateral talks.

The United States has diplomatic relations with Syria but withdrew its ambassador to Damascus in early 2005 and has not had high-level contacts for the past two years.

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