The Vatican said it was "deeply pained" by the outbreak of war and deplored the interruption of efforts to bring about a peaceful solution.
In the Vatican's first reaction to the start of the Iraq conflict, chief spokesman Joaquin Navarro-Valls criticised Iraq and the US-led coalition - but appeared to be harsher with Washington for the breakdown of diplomacy.
"The Vatican is deeply pained by the latest developments in Iraq," Mr Navarro-Valls said.
"On the one hand it laments the fact that the Iraqi government did not accept the resolutions of the United Nations and the appeal by the Pope himself, which asked for the country to disarm," the spokesman said.
"On the other hand, it deplores the interruption of the path of negotiations, according to international law, for a peaceful solution to the Iraqi drama".
It said the Vatican's embassy in Baghdad would stay open despite the conflict to help oversee Catholic charitable efforts for Iraqis.