Iranian President Mahmoud Ahmadinejad has offered Iraq full support in stabilising the security situation in the country.
Iran-Iraq relations have improved since Saddam Hussein's overthrow
He made the remarks in Tehran after talks with the Iraqi Prime Minister Nouri Maliki.
Speaking to reporters after their meeting, Mr Ahmadinejad said "Iraq's security is Iran's security".
Mr Maliki is making his first official visit to Iran since he took office in May.
"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.
Mr Maliki said his discussions with Mr Ahmadinejad had been positive. "Even in security issues there is no barrier in the way of co-operation."
Few concrete details of their talks have emerged, except that an agreement covering political, security and economic co-operation was signed.
After fighting a long war in the 1980s, the relationship between Iran and Iraq has improved since the fall of Saddam Hussein in 2003.
Many of Iraq's new Shia leaders have close ties to neighbouring Iran.
Mr Maliki lived in Iran during the 1980s when Saddam Hussein was in power in Baghdad.
The United States has accused Iran of destabilising Iraq by backing Shia militant groups there.
Last year, Britain said explosive devices used to attack British troops in southern Iraq had "Iranian elements".
Iran has rejected these allegations.
Mr Maliki is due to meet Iran's Supreme Leader, Ayatollah Khamenei, on Wednesday.
During his visit, he is expected to also press for the release of six Iraqi border guards who were seized last week after a reported exchange of fire with Iranian forces.