BAGHDAD - The United States plans to "go after" what it said were networks of Iranian and Syrian agents in Iraq, U.S. ambassador Zalmay Khalilzad said Monday.

"We're going after their networks in Iraq," he told a news conference, as he laid out the new U.S. and Iraqi strategy to end sectarian violence at what Khalilzad called a "defining moment" for Iraq.

U.S. forces are holding five Iranians following a raid on an Iranian government office in Arbil last week - the second such operation in recent weeks.

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Meanwhile, U.S. Defense Secretary Robert Gates said Monday that increased U.S. military activity in the Persian Gulf is meant to counter "very negative" behavior by Iran and undercut its belief that American forces are overcommitted in Iraq.

Gates said the time is not right for diplomatic talks with Iran, but left open that possibility for the future.

After meeting with senior officials at NATO headquarters, Gates was asked at a news conference what was behind the Bush administration's decision to deploy a Patriot missile battalion and a second aircraft carrier to the Gulf region - moves announced in connection with a further buildup of ground troops in Iraq.

He noted that the United States has taken a leading role in Gulf security for many decades.

"We are simply reaffirming that statement of the importance of the Gulf region to the United States and our determination to be an ongoing strong presence in that area for a long time into the future," he said.

Gates, who as recently as 2004 publicly called for diplomatic engagement with Iran, said the situation has changed. In 2004 Iran was concerned by the presence of U.S. forces on its eastern and western borders, in Iraq and Afghanistan. More recently, the Iranian government has come to see it differently, he said.

"The Iranians clearly believe that we are tied down in Iraq, that they have the initiative, that they are in position to press us in many ways," he said. "They are doing nothing to be constructive in Iraq at this point."

He added, "And so the Iranians are acting in a very negative way in many respects. My view is that when the Iranians are prepared to play a constructive role in dealing with some of these problems then there might be opportunities for engagement."

Besides concerns that Iran is intent on using its nuclear program to build weapons, the U.S. has accused Tehran of fueling violence in Iraq and not doing enough to block foreign fighters from entering Iraq over its border.