"Our hearts and our prayers go out to them," she told "Good Morning America" today. "The U.S. is going to be there, not just for today, but for the weeks and months and years ahead."
Clinton declined to estimate how many people died in the earthquake, but said it is estimated the disaster affected 3 million people.
World Vision spokeswoman Maggie Boyer said the city is overwhelmed by the number of people who don't know what to do next.
"I think the city is beginning to show signs of real displaced people who are just roaming and roving throughout the streets unsure of where to go," she said.
President Obama has promised a full military and civilian response to Haiti.
The U.S. Navy aircraft carrier Carl Vinson and the destroyer USS Higgins are expected to reach Haitian waters today, along with members of the 82nd Airborne Division. Up to 1,000 U.S. soldiers will arrive in Haiti over the next 24 hours and 2,200 will be deployed by Sunday. Several other ships, including a large hospital ship have been prepped for possible deployment.
Among the first of the oustide relief to reach Haiti was an Air China plane that was able to land at the Port-au-Prince airport. More than 50 members of a search and rescue crew along with trained dogs quickly got to work. China said the plane also carried medics, seismological experts and 10 tons of food, medicine and other supplies. Many of the Chinese sent to Haiti were experienced in dealing with such a disaster, having worked on China's 2008 quake that killed nearly 90,000 people.
They joined rescue units from France and Spain as well as Fairfax, Va., which had arrived on Wednesday.
Clinton called the earthquake particularly devastating because it came at a time when Haiti was just starting to rebuild it's infrastructure and economy for a series of devastating hurricanes in 2008.
New buildings had been going up and businesses were returning to Haiti, Clinton said, all part of a plan Haitian President Rene Preval had brought to the United States and the United Nations.
"We have seen this cycle of hope and despair so many times," Clinton said. ""Perhaps with this increased attention, we really can help Haiti rebuild."
"Thankfully the people of Haiti are a resilient people," she said.
Tuesday's earthquake was the ninth major natural disaster to slam Haiti in nine years.
So far there have been no reports of widespread looting or violence, but those on the ground worry that if aid doesn't start to reach the people soon, the fear and desperation may cause a wave of unrest.
Boyer said the sense of calm and order among even the most hard hit victims has been a very encouraging sign.
"We are just really trusting that that situation will remain the same," she said.
Raymond Joseph, the U.S. ambassador to Haiti, told "Good Morning America" today that as of now the U.S. military has not been asked to help provide security around Port-au-Prince, but rather to focus on helping its distraught, injured and hungry residents.
"[We] will probably be able to clear the debris from the roads," he said. "Once that is done we'll see more coordination on the ground."