U.S. Ambassador to Iraq Ryan Crocker was answering a question about why a more forceful response wasn't made to the upswing in violence and attacks on the Green Zone -- in which U.S. soldiers were killed -- after having "laid out a fairly persuasive case for Iranian involvement" in the attacks.

"These Iranian-provided rockets are coming down on their seat of federal government as well as some of our facilities," Crocker said during a joint news conference with Army Gen. David Petraeus in Washington Thursday. "We have told the Iraqis that we're prepared to move to another round of direct security talks if the Iraqis think that's a step they want to take."

If the Iranians were serious about negotiations, "we're certainly going to point out what we know of what they're doing to undermine Iraqi security and make the very strong point that they need to bring their actions into conformance with their policy," Crocker said.