US Ambassador Zalmay Khalilzad urged Iraq's neighbours to do more to stop the flow of fighters, weapons and sectarian propaganda contributing to violence in the country.
"Iraq's near neighbours and other friends can certainly do more to help in this transition toward stability and prosperity for Iraq," Mr Khalilzad said in a speech to a major conference in Baghdad.
"I urge all neighbors to categorically reject the principle that selective violence against certain categories of Iraqis or against Coalition and Iraqi security forces is acceptable," he said.
"Iraq's neighbours can only be counted as providing true and honest 'support' to Iraq's stability if they act decisively to halt the flow of fighters, weapons, and other lethal support to militias and other illegal armed groups, and cease sectarian rhetoric and other propaganda that could incite violence."
Mr Khalilzad said the future of Iraq and the Middle East was the defining issue of the moment.
"No country represented at the table would benefit from a disintegrated Iraq; indeed, all would suffer badly," he said. He hoped their presence indicated they were "ready to take concrete, constructive actions" to support Iraq.
Iraq called today's meeting to enlist support to end violence four years after the US invasion. It was also a rare opportunity for officials from Washington and Tehran to meet at a time of growing tension over Iran's nuclear ambitions.
Iranian delegates demanded the release of several Iranians captured by US forces in Iraq, a source at the talks said.
US soldiers seized five men Tehran says are diplomats in a raid on an Iranian government office in the northern Iraqi city of Arbil on January 11th.
Apparently responding to the Iranians, Mr Khalilzad said: "The Coalition does not have anyone in detention who is a diplomat."
"The U.S. and Coalition forces will continue to take appropriate steps under our mandate to protect Iraq's citizens, and our soldiers, from foreign elements who are contributing to violence in Iraq," he said.
"No one should doubt our resolve in this regard."