Hundreds of thousands of people have fled their homes since the U.S.-led invasion of Iraq three years ago, say humanitarian aid groups, and a recent spike in those numbers is leading some to question the price for success in the U.S. mission there.

"The implications on plans for a kind of unified Iraq or national state over the long term are very worrisome," said Tony Sullivan, chief Middle East correspondent for Strategic Forecasting Inc., a security consulting intelligence agency.

But U.S. officials are warning against exaggerating the magnitude of the crisis.

"There is ... indication of displaced persons inside of Iraq," Maj. Gen. Rick Lynch, spokesman for the U.S. command, told reporters in Baghdad April 27. He disputed figures by Iraqi officials, and added that many Shiites and Sunnis moving around Iraq are doing so for personal reasons.

"We're not seeing internally displaced persons at the rate which causes us alarm," Lynch said.

In late April, Iraqi Vice President Adel Abdul Mahdi announced that more than 100,000 individuals have been displaced, but did not clarify his sources or say whether that figure included displacements before the Feb. 22 bombing of a revered Shiite shrine in Samarra. Other numbers have ranged from 65,000 to 100,000 people who have fled their homes to other places inside Iraq since the mosque bombing.

As far as Iraqis leaving the country, estimates range from 500,000 to 1 million since the war in Iraq began in 2003.

"It is an ominous sign, signaling what could happen if sectarian violence spreads throughout the country, and the country descends into civil war,” said Roberta Cohen, a world refugees and foreign policy scholar for the Brookings Institution in Washington, D.C.

Cohen said the variance in the reported numbers stems in part from the fact that no real count of displaced persons was done before the Feb. 22 bombing — an event that is considered to have touched off the sectarian violence throughout much of central Iraq.

Since then, reports of armed Shia militias, death squads, suicide bombings and reprisal murders against Sunni and Shia civilians have been consistent in the daily headlines.

“The numbers of those displaced by sectarian violence are not large given Iraq’s past history,” Cohen said, noting that during Saddam Hussein's regime, the dictator deliberately stripped many in the Kurdish minority community of their homes.

“What is of great concern is that the numbers seem to be growing. This begins to add up,” she said.

Representatives of relief and human rights organizations on the ground in Iraq and neighboring Jordan, which has taken in the bulk of fleeing Iraqis since the war began, say the success of the U.S. mission depends on whether authorities can minimize displacement and migration out of the country.

"We're very concerned because the displacement doesn't seem to be subsiding," said Dana Graber, IDP monitoring director for the International Organization for Migration in Iraq, which is tracking displacement trends and working with non-governmental organizations on relief efforts. IOM estimates that 70,000 people have been displaced since the February mosque bombing, and more than 165,000 individuals have left their homes since the war began.

Bill Frelick, the refugee policy director of Human Rights Watch who just returned from a trip to Jordan to assess the refugee situation from there, said his group estimates at least 750,000 Iraqi refugees are living in that country today.

Like the IDP figures, hard and fast numbers of refugees are elusive, but relief workers are also seeing an ethnic mix.

"Religious minorities are finding themselves more vulnerable," and the recent spate of violence has exacerbated ethnic tensions and has also put Iraqi contractors working with Americans at risk, Frelick said.

Most agree the currently displaced individuals are a mix of Iraq's population — mostly Sunni, Shia, Arab Christians and Palestinians. Palestinians and Iranian Kurds attempting to leave the country are having the most difficult time crossing the border out of Iraq, Frelick said.

Frelick credited the Jordanians for being "flexible," and called the people there humane, but he said it is unclear how many more Iraqis that country can take.

"It's basically a burden on a country that does not have oil, it hardly has water and doesn't have the resources to deal with this problem," Frelick said.

In pre-war planning, coalition forces had prepared for a potential refugee crisis after the initial invasion, but it never happened. They were not necessarily prepared for that circumstance three years later.

"This alarmingly high and growing number of refugees is further evidence of the failure of U.S policy in Iraq as well as jeopardizes future hopes for stability there," said Dahr Jamail, an American journalist who has spent a lot of time in Iraq and has been critical of the war policy. "Alarm bells should be ringing in the Bush administration."

State Department officials deferred comment on the refugee situation to the Iraqi government. However, the State Department and the U.S. Agency for International Development have been aiding displaced persons.

Arthur Keys, president of International Relief and Development, which has about 150 mostly Iraqi staff working with displaced persons in Iraq, said the Iraqi government, the United Nations, USAID and State Department have all been responsive with direct assistance.

“There has been a concrete response,” he said. “Whether it has been enough or not I don’t know.”

Sullivan said the migration trends primarily in the Baghdad, Anbar and Diyala provinces — Sunnis to the west and north, Shia toward the south, and Kurds solidifying their autonomy in northern Iraq — hint at a three-state solution, something for which the United States has not bargained.

"It's definitely bad news," Sullivan said. "Iraq is divided into three parts and it's divided more today than it was yesterday and more than the day before."

Some lawmakers and foreign policy analysts have recently called for supporting a partitioning of Iraq instead of a centralized government in Baghdad.

Sen. Joseph Biden, D-Del., proposed in April that a "third option" would be to divide Iraq into autonomous Sunni, Shia and Kurdish regions. Critics say this option would inflame ethnic tensions and may even encourage ethnic cleansing across Iraq.

Cohen suggested it's too soon to believe the migration of people will result in a de facto partitioning of Iraq.

“There are certainly enough parts of the country that are still mixed,” said Cohen. “You would need some kind of decision-making, at the government level, of relocating people. The (Iraqi) government is supportive of a unified country, at least at this point, and so is the U.S."