Violence has diminished in about half of the areas of Baghdad where U.S. troops have applied a new counterinsurgency strategy since February, the top U.S. commander in the Middle East said Thursday.

Navy Adm. William Fallon, commander of U.S. Central Command, told the Senate Armed Services Committee that while he is encouraged by signs of improved security in the Iraqi capital, he is deeply worried by insurgents' ability to continue carrying out spectacular suicide bombings.

"We literally hold our breath," in anticipation of the next big bombing, he said.

Fallon also expressed skepticism about the ability of Prime Minister Nouri al-Maliki and other government leaders to achieve the degree of political reconciliation among rival sects that Fallon said are urgently needed.

"They are not moving, in my opinion, fast enough," he said.

The admiral also said he was disturbed by a Washington Post report Monday that a department of Maliki's office is playing a leading role in the arrest and removal of senior Iraqi army and national police officers, some of whom may have moved too aggressively against violent Shiite militias.

Fallon said he could not confirm the particulars of the report.

"The reports that this Office of the Commander in Chief is in fact working behind the scenes to set up a parallel organization to the Ministry of Defense is disturbing," he said, because it can undermine the confidence of the Iraqi people in their government leadership.

While noting that the flow of additional U.S. troops to Baghdad will not be completed until June, Fallon said positive results already are evident.

"My sense is that, at first glance, about half of the areas are dramatically improved with levels of violence lessened, but there's a distinctly active area" in western Baghdad where violence levels are up, Fallon said.

Fallon's comments — his first to the Senate panel since his Jan. 30 confirmation hearing — came amid a heated debate in Congress over how long to keep U.S. troops in Iraq.

President Bush this week vetoed $124.2 billion legislation ordering troops to begin leaving Oct. 1. Failing on Wednesday to gain a two-thirds majority needed to override the veto, Democrats were expected to begin negotiations Thursday with top White House aides on the next step.