Brahimi said that the international community's inaction on the conflict in Syria had precipitated the crisis in Iraq.
"It is a well-known rule: a conflict of this kind (in Syria) cannot stay confined within the borders of one country," Brahimi told AFP.
The international community "unfortunately neglected the Syrian problem and did not help to resolve it. This is the result," said Brahimi, who resigned from his post as UN-Arab League representative to Syria in May.
The Algerian statesman briefly served as UN special envoy for Iraq in 2004, following the US-led invasion that toppled the regime of executed dictator Saddam Hussein.
He stressed that he had told the UN Security Council in November that the Islamic State of Iraq and the Levant, the group spearheading the offensive in Iraq, was "ten times more active in Iraq than in Syria".
Brahimi added that Iran, a key ally of the government in Baghdad, had "its place" in the region, pointing to the "de facto cooperation between the United States and Iran" on the crisis in Iraq.