General Tommy Franks' statement coincided with calls from countries demanding a United Nations mandate before committing troops to a multilateral peacekeeping mission in Iraq.
Franks told the House Armed Services Committee on Thursday it would be unwise to think difficulties faced by "the coalition will go away in one month or two months or even three months".
"I anticipate we'll be involved in Iraq in the future," Franks said in a testimony ahead of his retirement from the military on Monday. "Whether that means two years or four years, I don't know."
Franks also warned against reducing the strength of US troops in Iraq and the possibility of anti-US attacks ending in the near future.
Sixty-five US soldiers have died in Iraq since the end of major combat over two months ago. Many of them were killed in ongoing Iraqi resistance operations against US occupation.
NATO, US role
Franks' assessment and the mounting US casualty toll in Iraq have raised fears of a drawn-out and costly post-war stay in Iraq.
The US Senate unanimously called on the White House to consider asking for NATO and UN troops to be sent into Iraq.
In a 97-0 vote on Thursday, senators said President George W. Bush "should consider requesting formally and expeditiously that NATO raise a force for deployment in post-war Iraq similar to what it has done in Afghanistan, Bosnia and Kosovo".
"We need more forces ... and we have to make it clear that we're not a force of occupation," Senator Joseph Biden, top Democrat on the Senate Foreign Relations Committee, said.
"There's a need to internationalize this and to bring in NATO in particular," he added.
Iraqi anti-occupation protests
are on the rise
But US Secretary of State Colin Powell cautioned on Thursday against illusions about the amount of help the US and Britain would get from allies.
"I can't give you the exact number of nations or how many troops are going to be committed," Powell said.
The newspaper USA Today also poured cold water on expectations of quick foreign help.
It said, on Thursday, India, Pakistan and Portugal, which the Pentagon had hoped would deliver between 25,000 and 30,000 soldiers, were now insisting the United Nations approve a UN mandate for the force first.
Germany has also set strict conditions before it would consider sending to troops to Iraq to take part in a multinational peacekeeping mission.
Berlin wants a "request (for troops) from a legitimate Iraqi interim administration and a clear UN mandate" before it even discusses the issue, a government spokesman said on Friday.
US Defense Secretary Donald Rumsfeld said Wednesday he had asked Germany and France to share the cost and responsibility for the ongoing military occupation in Iraq.
French Foreign Minister Dominique de Villepin said in an interview on Thursday France would only join the force if it had UN backing.
But Spain on Friday said it would deploy 1,300 military personnel to join the Polish-led force that would take over security in southern Iraq.