The U.S. military's plans called for the arrival of some 7,500 more forces, to augment around 5,000 already on the ground or on naval vessels offshore.
Troops are airlifting emergency supplies and the injured, providing logistical support, managing the Port-au-Prince airport and standing by to help provide security amid scattered reports of looting and violence in the capital.
Kenneth Merten, the U.S. ambassador to Haiti, said in a television interview this morning that the security situation was "obviously not perfect" but could be managed.
Merten said the Haitian police and U.N. peacekeepers had suffered losses that had impaired their ability to provide law and order.
"This is not a perfect law-and-order situation here even in the best of times," Merten said from Haiti on NBC's "Today" show. "While we're watching it closely, we're concerned about it, we're monitoring carefully, we don't believe it's anything that's unmanageable at this point."
He said U.S. troops would play a backup security role if neither Haitian police nor U.N. forces were able to maintain order. Merten said U.S. troops had stepped in "on a couple individual occasions."
In a sign that the relief effort was picking up steam, the U.N. World Food Program said today it would hand out ready-to-eat meals to 95,000 Haitians, an increase over the 67,000 people fed a day earlier.
The agency plans to hand out 10 million meals, plus rice and high-energy biscuits, during the next week and estimates that it will need to provide 100 million meals during the next 30 days.
Josette Sheeran, executive director of the Rome-based agency, said aid workers were coordinating with the U.S. military and Haitian aviation authorities at the Port-au-Prince airport to give priority to humanitarian flights and ensure a more orderly flow of goods.
Some aid donors have complained that their flights were turned away from the damaged airport, which since the earthquake has been run by the U.S. military.
Sheeran said the ability of relief groups to get help to victims is "improving hour by hour," though the need is still massive. The list of immediate priorities, she said, included clearing roads; repairing the capital's seaport, which was forced to close due to quake damage; guaranteeing security at food handout points; and getting more trucks and helicopters to deliver goods.
The agency said another 43 metric tons of high-energy biscuits were being flown today from El Salvador to Port-au-Prince and that it was planning to shuttle 10,000 gallons of much-needed diesel fuel per day from neighboring Dominican Republic.
World Food Program officials said they would have to find new warehouses for food supplies in Port-au-Prince because of damage to previous storage facilities caused by aftershocks on Sunday.
Meanwhile, European Union bodies and member states have offered more than 400 million euros, or about $576 million, in relief and recovery aid, Reuters news agency reported.
Former President Clinton, the U.N. special envoy to Haiti, today was to travel to the Caribbean nation. He is the latest in a string of U.S. and U.N. officials to visit since the 7.0 earthquake hit Tuesday, causing widespread destruction and killing perhaps 100,000 or more people, according to preliminary estimates by Haitian officials.
Clinton planned to meet with Haitian officials and aid workers and help to deliver emergency supplies: food, water, solar-powered flashlights, portable radios and generators, according to a statement from the Clinton Foundation.
Clinton and former President George W. Bush have been tapped by President Obama to raise donations for earthquake relief. The website is ClintonBushHaitiFund.org.
"As U.N. special envoy for Haiti, I feel a deep obligation to the Haitian people to visit the country and meet with President [René] Préval to ensure our response continues to be coordinated and effective," Clinton said in the statement, issued Sunday.