The United States has said it plans to "go after" what it says are networks of Iranian and Syrian agents in Iraq.
Washington also told the Iraqi public not to expect an instant improvement in security in the capital, Baghdad.
This came in a joint press conference given by Zalmay Khalilzad, the US ambassador to Iraq, and General George Casey, the outgoing US military commander in Iraq, in Baghdad on Monday.
"We're going after their networks in Iraq," Khalilzad told the news conference.
"We will target these networks in the ... expectation of changing the behaviour of these states," Khalilzad said as he laid out the new US and Iraqi strategy to end sectarian violence, by both Sunnis and Shia, at what he called a "defining moment" for Iraq.
On his return, Casey said the latest US plan to beef up its forces and secure Baghdad had "no guarantees of success" and that it was "not going to happen overnight".
He said he did not expect significant results until the summer and autumn, for the first time putting a timeframe around the new plan that was announced on Wednesday by George Bush, the US president.
"As with any plan, there are no guarantees of success, and it's not going to happen overnight, but with sustained political support and the concentrated efforts on all sides, I believe that this plan can work," Casey said.
"The plan is Iraqi-conceived and Iraqi-led, but we are involved in every step."
The US general stressed that rogue Shia forces believed to be behind much of the country's chronic sectarian violence would be targeted in the new plan.
"Militias will not be allowed to be an alternative to the state or to provide and to take on local security around the country," Casey said.
Meanwhile, car bomber killed at least five people and wounded 28 more in an attack on an office of the Kurdistan Democratic Party (KDP) in the northern city of Mosul on Monday, police and a Kurdish official said.
Police said the number of civilian casualties was four dead and 26 wounded.
Lieutenant Ahmed Zebari, a military officer with links to the KDP, said inside the office one more person was killed and another two wounded. They were taken to a different hospital.
The KDP is one of the two ruling parties in Kurdistan in northern Iraq. With a mixed population of Arabs and ethnic Kurds, Mosul has long been a flashpoint of violence.