WASHINGTON — As the number of U.S. troops in Haiti and aboard a small armada floating offshore build toward 18,000, the question of how and when they will leave remains unanswered.

WASHINGTON — As the number of U.S. troops in Haiti and aboard a small armada floating offshore build toward 18,000, the question of how and when they will leave remains unanswered.

While past humanitarian missions, most notably in Somalia in the 1990s, have changed into protracted — and bloody — “peacemaking” exercises, experts said there are many reasons Haiti is unlikely to turn into a quagmire for U.S. forces.

If anything, said a half-dozen officials with experience in relief and peacekeeping operations, the danger is that the troops, ships and helicopters will leave too soon, before security is re-established. With wars in Afghanistan and Iraq, the Obama administration has little appetite for an extended military mission in the Caribbean.

“The risk is the opposite: that they will leave too quickly, and we will have chaos,” said Andrew Natsios, who led the U.S. Agency for International Development from 2001 to 2005. “They’ve got their hands full in Iraq, Afghanistan, Pakistan. The U.S. military does not want to do this, in terms of anything beyond the humanitarian response.”

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Haiti’s 7.0-magnitude earthquake Jan. 12 devastated the capital, Port-au-Prince, left up to 200,000 dead and crippled an existing U.N. peacekeeping mission and the country’s government.

There are about 13,000 U.S. military personnel in Haiti — about 4,000 ashore and 9,000 aboard ship — and that will grow to 17,000 to 18,000 by this weekend with the arrival of a second Marine Expeditionary Unit, said Marine Col. David Lapan, a Pentagon spokesman. Twenty-two ships and 66 military helicopters are participating in the relief effort, he said.

“There hasn’t been an impact to this point” on operations in Iraq and Afghanistan, Lapan said.

U.S. officials, meanwhile, said a new accord with the United Nations formally gives the U.S. military a supporting role in international relief efforts in Haiti, but keeps it in charge of the nation’s airspace, ports and roads.

The pact gives Haitian authorities and the U.N. peacekeeping mission in Haiti primary responsibility for maintaining law and order, but it grants the U.S. military broad scope to intervene in civil disturbances, subject to a request by Haitian authorities.

The agreement says U.S. authorities will establish a U.S. Joint Task Force Haiti to support the international humanitarian response. Friday’s pact formalizes powers the U.S. military has been exercising in Haiti. It is also intended to clarify the division of powers among the Haitian police, the U.N. peacekeeping mission and the U.S. military.

The agreement states the U.S. military will remain under its own command.

If all goes as planned, U.S. troops will begin leaving Haiti after United Nations agencies and private aid groups are ready to fully take on the task of recovery and rebuilding. A U.N. peacekeeping force, which is being enlarged with 2,000 more peacekeepers and 1,500 police officers, will provide security alongside Haitian forces.

“As we get through this initial crisis, as those other organizations bring up their capacity, we will work with all those organizations to determine when the right time is to transition our capabilities out of Haiti,” Air Force Gen. Douglas Fraser, the commander of the military’s Southern Command, said Thursday.

Neither the White House nor the Pentagon has spelled out the details of when and how that will happen, however.

Large-scale violence, which has been sporadic, or thousands of desperate Haitians taking to the seas to try to reach U.S. shores could upend the Obama administration’s plans.

It also remains to be seen who will coordinate the task of rebuilding Haiti, which will take years, cost billions of dollars and involve hundreds of agencies and charities.

Material from The Washington Post is included in this report.