Addressing a news briefing after talks with visiting Iraqi Prime Minister Nuri Kamil Al-Maliki, Ahmadinejad said that "Iraq's security is Iran's security."
Al-Maliki said the two leaders had a "good discussion" and said he saw "no barrier" to cooperation between the two countries, "even in security issues."
Al-Maliki is in Iran on a two-day visit aimed at examining ways to improve relations between the two neighbors.
The Iraqi prime minister, who is traveling with several cabinet ministers and lawmakers, is also due to meet with Supreme Leader Ali Khamenei and other Iranian officials.
The White House reacted warily to Iran's pledge, suggesting that Tehran was "part of the problem" in Iraq. Spokesman Tony Snow called on Tehran not to finance "separatist and terrorist groups" in the country.
Officials in Baghdad have in the past said they suspect Tehran of fostering unrest among Iraq's Shi'ite majority.
(IRNA, IRIB, AFP)