ASHINGTON - Adm. William Fallon, who is poised to become the top American commander in the Middle East, says the United States miscalculated the ability of Iraqi forces to take control and underestimated the enemy's persistence.

Navy Adm. William J. Fallon, President Bush's nominee to head U.S. Central Command replacing Gen. John Abizaid, listens as Bush, not pictured, makes a statement to reporters as he meets with the joint chiefs and combatant commanders in the Cabinet Room of the White House in Washington Wednesday, Jan. 24, 2007. [AP]

"Securing the stability of the country has been more difficult than anticipated," Fallon said in a written statement to the Senate Armed Services Committee. "Our ability to correctly assess the political, economic and security situation in Iraq has been lacking."

Fallon's remarks were submitted in advance of a confirmation hearing Tuesday. Fallon, who commands troops in the Pacific region, has been tapped to replace Army Gen. John Abizaid as head of the US Central Command.

In addition Tuesday, the Senate Foreign Relations Committee was expected to consider the nomination of John Negroponte, the first director of national intelligence, to become deputy secretary of state.

Fallon and Negroponte's confirmations were not expected to rouse Senate protests, despite bitter opposition in Congress to Bush's plan to send 21,500 additional troops to Iraq.

Public sentiment has turned strongly against a war that has dragged on for nearly four years with more than 3,000 American dead and violence unabated by insurgents and sectarian militias.

In remarks prepared for a speech Tuesday, Sen. Bill Nelson, D-Fla., a member of the Armed Services Committee who recently returned from a trip to the region, said only another 200,000 or 300,000 US troops would make a substantial difference in Iraq.

"Based on everything I saw last month, and based on my conversations with Iraqi officials, our own military leaders and rank-and-file soldiers, I am convinced more troops won't end the sectarian violence," Nelson said.

Nelson also was expected to deliver a sharp rebuke of Iraqi Prime Minister Nouri al-Maliki. Nelson said al-Maliki "either lacks the will, or the nerve, to take on the Shiite militias."

Last week the Senate approved, 81-0, Bush's nomination of Army Lt. Gen. David Petraeus to head the Iraq war. Petraeus would work alongside Fallon, who would oversee military operations throughout the region, including Afghanistan.

During Petraeus' Jan. 23 confirmation hearing, senators questioned him on how Bush's new strategy would work and whether Congress should weigh in with a resolution of disapproval. Petraeus said the situation in Iraq was "dire" but not hopeless.

In his written remarks, Fallon told the Senate panel that the US has "relearned" the need to hold secure an area "until Iraqi security forces and local political and economic activity have provided essential confidence to the population."

Fallon also said he believes the Pakistan government should do more to prevent al-Qaida operatives from crossing its border into Afghanistan and that Iran remains a serious threat to the region.

However, Fallon said Tehran was unlikely to produce a nuclear weapon until "mid-next decade."

"I sense that our allies in the region are more concerned about the potential threat posed by Iran now than at any time since the Iran-Iraq War," he wrote.

Petraeus is to arrive in Baghdad to take over for Gen. George Casey as the top US commander in Iraq next week, a defense official said Monday. Casey, tapped to become the next Army chief of staff, will face the Senate Armed Services Committee on Thursday.