By air and increasingly by sea, a massive evacuation of Americans to take them out of harm's way in Lebanon was under way Monday.

An estimated 25,000 Americans are there. Some 15,000 have registered with the U.S. Embassy in Beirut, but not all evidently are trying to get out. "Our planning assumptions are on the order of thousands," State Department spokesman Sean McCormack said. "You don't actually know how many people are going to want to leave until you actually start the larger-scale operations."

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The operation began slowly. By late Monday only 64 were known to have departed.

U.S. government officials, basing their estimates on similar situations in the past, say the range of Americans lining up to leave could vary from 10 percent of all those in the country all the way up to 100 percent.

Most other countries have a far less difficult evacuation task since there far fewer of their people in Lebanon. Convoys of buses have been used effectively.

But the U.S. government is discouraging travel by land to the border of neighboring Syria. Two of the three major roads have been bombed severely, Maura Harty, assistant secretary of state for consular affairs told CNN. And some Americans who got to the border were denied entry by Syria.

"We did not think that was a wise way to counsel people to leave the country," she said.

At the Pentagon, spokesman Bryan Whitman said a commercial ship, the Orient Queen, had been contracted to ferry evacuating Americans to Cyprus. He said it could carry 750 people at a time.

A U.S. Navy destroyer, the USS Gonzales, will be available to escort the Orient Queen, he said.

The French lent a hand. In Paris, the foreign ministry said 50 U.S. citizens were evacuated with 800 French citizens and 400 other Europeans on a Greek ferry, Iera Petra, chartered by the French government.

Most of the first Americans to depart were removed by U.S. helicopters, some of which flew to a British base on Cyprus.

At the State Department, McCormack said the cost of a massive evacuation was beyond U.S. resources. He said evacuated Americans would be asked to pay commercial rates, and if they did not have the money to promise to pay in the future.

"Everybody who wishes to leave will be able to leave," he promised.

The U.S. Embassy advised Americans to carry a valid passport, a birth certificate and marriage or other civil documents. Each traveler is limited to one suitcase weighing up to 30 pounds. Pets will not be allowed to travel.

The embassy, itself, is not being evacuated, Harty said in an ABC News interview. But dependents of U.S. personnel who have chosen to leave will be able to depart, she said.

Two organizations, one Arab-American and the other Muslim-American, criticized the slow start and that the United States was not promoting a cease-fire.

"The absence of American leadership to secure a cease-fire and protect its own citizens is appalling," said James Zogby, president of the Arab-American Institute.

Nihad Awad, executive director of the Council on American-Islamic Relations, said "the highest duty of any president is to protect the lives of Americans."

Many of the U.S. citizens in Lebanon are Arab-American making regular summer visits to family members.