( CNN ) -- A major earthquake struck southern Haiti on Tuesday , knocking down buildings and power lines and inflicting what its ambassador to the United States called a catastrophe for the Western Hemisphere 's poorest nation . Several eyewitnesses reported heavy damage and bodies in the streets of the capital , Port-au-Prince , where concrete-block homes line steep hillsides . There was no estimate of the dead and wounded Tuesday evening , but the U.S. State Department has been told to expect `` serious loss of life , '' department spokesman P.J . Crowley told reporters in Washington . `` The only thing I can do now is pray and hope for the best , '' the ambassador , Raymond Alcide Joseph , told CNN . Pictures sent to CNN 's iReport show homes and small businesses in Haiti that have collapsed . Are you there ? Submit an iReport . The magnitude 7.0 quake -- the most powerful to hit Haiti in a century -- struck shortly before 5 p.m. and was centered about 10 miles ( 15 kilometers ) southwest of Port-au-Prince , the U.S. Geological Survey reported . It could be felt strongly in eastern Cuba , more than 200 miles away , witnesses said . Experts warned of earthquake risk on Haiti . Mike Godfrey , an American contractor working for the U.S. Agency for International Development , said `` a huge plume of dust and smoke rose up over the city '' within minutes of the quake -- `` a blanket that completely covered the city and obscured it for about 20 minutes . '' Witnesses reported damaged buildings throughout the capital , including the president 's residence and century-old homes nearby , and The Associated Press reported that a hospital collapsed . President Rene Preval is safe , Joseph said , but there was no estimate of the dead and wounded Tuesday evening . He said an official of his government told him houses had crumbled `` on the right side of the street and the left side of the street . '' `` He said it is a catastrophe of major proportions , '' Joseph said . Impact Your World : How you can help . Frank Williams , the Haitian director of the relief agency World Vision International , said the quake left people `` pretty much screaming '' all around Port-au-Prince . He said the agency 's building shook for about 35 seconds , `` and portions of things on the building fell off . '' `` None of our staff were injured , but lots of walls are falling down , '' Williams said . `` Many of our staff have tried to leave , but were unsuccessful because the walls from buildings and private residences are falling into the streets , so that it has pretty much blocked significantly most of the traffic . '' Read what people in Haiti are saying via social media . Haiti 's government is backed by a U.N. peacekeeping mission established after the ouster of former President Jean-Bertrand Aristide in 2004 . The headquarters of the U.N. peacekeeping mission in Port-au-Prince collapsed , a U.N. official told CNN . There was no immediate report of any dead or wounded from the building , but Alain Le Roy , the undersecretary-general for peacekeeping operations said of the 9,000-member , Brazilian led-force , `` For the moment , a large number of personnel remain unaccounted for . '' Outside the capital , several people were hurt when they rushed to get out of a school in the southwestern city of Les Cayes , said the Rev . Kesner Ajax , the school 's executive director . Two homes in the area collapsed and the top of a church collapsed in a nearby town , he said , but he did not know of any fatalities . Les Cayes , a city of about 400,000 people , is about 140 miles ( 225 kilometers ) southwest of Port-au-Prince . The quake took place about 6 miles ( 10 kilometers ) underground , according to the USGS -- a depth that can produce severe shaking . At least 10 aftershocks followed , including two in the magnitude 5 range , the USGS reported . Appeals for aid after quake strikes Haiti . Jean Bernard , an eyewitness in Port-au-Prince , told CNN the city had no electricity Tuesday evening . The first quake lasted 35 to 40 seconds , he said . `` A lot of houses [ and ] buildings went down , and people are still running all over the streets , '' Bernard said . `` People are looking for their wives , looking for their husbands and their kids . It 's scary . '' Luke Renner , an American staying in Cap-Hatien , a city nearly 100 miles north of Port-au-Prince , said he was sitting at his home when `` the whole world started to shake . '' `` It felt like our whole house was balancing on a beach ball , '' Renner said . `` We heard the whole community screaming and in an uproar during that whole 20- to 30-second window . '' `` I have n't seen any structural damage here , '' Renner continued . `` With the sun setting it may be difficult to tell . In the morning we 'll know for sure . '' Because of the earthquake 's proximity to the capital , and because the city is densely populated and has poorly constructed housing , `` it could cause significant casualties , '' said Jian Lin , a senior geologist at the Woods Hole Oceanographic Institution in Massachusetts . In Washington , President Obama said the U.S. government would `` stand ready to assist the people of Haiti . '' At the Pentagon , the U.S. military said humanitarian aid was being prepared for shipping , but it was not yet clear where or how it would be sent . A U.S. aviation source said the control tower at the Port-au-Prince international airport collapsed , possibly hindering efforts to fly relief supplies into the country . Secretary of State Hillary Clinton told reporters that Washington is offering `` our full assistance '' to Haiti . '' The deputy chief of the U.S. mission in Haiti , David Lindwall , told Clinton that he saw `` significant damage '' from the quake and said U.S. officials there expect `` serious loss of life , '' Crowley said . And Clinton 's husband , former President Clinton -- now the U.N. special envoy for Haiti -- said the world body was `` committed to do whatever we can to assist the people of Haiti in their relief , rebuilding and recovery efforts . '' The United States has been heavily involved in Haiti commercially , politically and militarily for most of the last century . U.S. intervention under Clinton restored Aristide to power in 1994 after a 1991 coup , and a U.S. jet hustled him out of the country again in 2004 following a rapidly spreading uprising against his government . The disaster is the latest to befall the country of about 9 million people , roughly the size of Maryland . It is the poorest country in the Western Hemisphere and among the poorest in the world . With people stripping the trees for fuel and to clear land for agriculture , the mountainous countryside has been heavily deforested . That has led to severe erosion and left Haitians vulnerable to massive landslides when heavy rains fall . Hurricane Gordon killed more than 1,000 people in 1994 , while Hurricane Georges killed more than 400 and destroyed the majority of the country 's crops in 1998 . And in 2004 , Hurricane Jeanne killed more than 3,000 people as it passed north of Haiti , with most of the deaths in the northwestern city of Gonaives . Gonaives was hit heavily again in 2008 , when four tropical systems passed through . In addition , a Haitian school collapsed in November 2008 , killing more than 90 people and injuring 150 -- a disaster authorities blamed on poor construction . Eighty percent of Haiti 's population lives under the poverty line , according to the CIA World Factbook . CNN 's Mallory Simon , Edvige Jean-Francois , Shasta Darlington , Deb Feyerick , Matt Smith , Pierre Meilhan , Mike Mount and Brandon Griggs contributed to this report .
NEW : Witnesses : Bodies in streets of capital ; buildings collapsed . NEW : U.N. mission headquarters reportedly collapsed ; some unaccounted for . Earthquake struck just off coast of capital Port-au-Prince on Tuesday . Americans seeking info about relatives in Haiti can call State Department at 888-407-4747 .