Iranian President Mahmoud Ahmadinejad met with Iraqi Prime Minister Nuri Al-Maliki in Tehran.
"Iran will give its assistance to establish complete security in Iraq because Iraq's security is Iran's security," President Ahmadinejad told reporters after talks with Mr Maliki.
Iran has emerged as one of the strongest allies of the Shiite-dominated government in Baghdad after the fall of Saddam Hussein, but has also drawn criticism for allegedly aiding Shiite militants.
The US military in August accused Iranian forces of training and providing weapons to Shiite extremists in Iraq in order to carry out violent attacks in the war-torn country.
The United States has regularly charged Iranians of funding and providing weapons for triggering violence in Iraq.
"We are still concerned about Iran's trying to support sectarian elements," White House spokesman Tony Snow said.
"But also we understand that Prime Minister Maliki is doing what he needs to as a head of state visiting a neighbour that has some power and certainly some influence, there being a lot of Shia Muslims, including Prime Minister Maliki," Mr Snow said.
"We will do everything we can not only to support the prime minister, but also to say to the Iranians, play a constructive role," said the spokesman.
"The most important thing that Iran can do is not be part of the problem by financing separatist and terrorist groups who are trying to undermine democracy in Iraq," Mr Snow said.
Shiites form a majority in Iraq, and their ascendancy has given the government in overwhelmingly Shiite Iran an increased say in the affairs of its neighbour -- although Tehran vehemently denies the US charges of interference.
"Iran supports the Iraqi government that has been created by the Iraqi people's votes, and strengthening a united and independent Iraq is in the interest of all the region," Mr Ahmadinejad said.