"We can't control nature, but we can and must be prepared for disaster when it strikes," he said in a statement at the White House.
"I urge citizens to listen closely to the instructions of local officials who have the full support of the federal government as they prepare for a potential tsunami and recover from any damage that might be caused," Obama said.
But an official at the Pacific Tsunami Warning Center says Hawaii "dodged a bullet."
It still will be about an hour before officials will be willing to give an all-clear in Hawaii, but there were no immediate reports of major damage around the Pacific rim. just tidal surges that reached up to about seven feet (2.1 meters) in some island chains.
Gerard Fryer, a geophysist for the tsunami center, defended the decision to urge evacuations of coastal areas, saying "better safe than sorry."
But in central Chile, a "state of catastrophe" was declared by President Michelle Bachelet, yet said the government has not asked for assistance from other countries.
Obama said he reached out toBachelet to let her know that the U.S. will offer support if the country needs it.
"The United States stands ready to assist in the rescue and recovery efforts and we have resources that are positioned to deploy should the Chilean government asks for our help," he said.
The jolt of the earthquake set off a tsunami that swamped a village on an island off Chile, then raced across the Pacific, setting off alarm sirens in Hawaii, Polynesia and Tonga.
The first waves in Hawaii are expected to hit shortly after 11 a.m. Saturday (4 p.m. ET) and measure roughly 8 feet at Hilo.
The state's civil defense authorities are "not taking any chances," a military spokesman at Pacific Command, which oversees the U.S. military presence in the region, told Fox News.
They have a "robust national guard force" trained specifically in natural disaster response, the spokesman said. The island's national guard is in charge for now, taking things very seriously, while Pentagon assets in the region are on standby, the spokesman said.
The Hawaiian islands are literally dotted with dozens of military installations that cover all four branches of the Armed Forces, offering what would appear to be sufficient help if needed.
The Navy, as a "precautionary measure," has sortied several ships to the Hawaii region, some leaving from Hawaii and some from San Diego, a Navy spokesman told Fox News. The military has also evacuated five military housing units there.
The Federal Emergency Management Agency said it is ready to help.
"FEMA and the Department of Homeland Security are closely monitoring the situation, and officials are in close contact with the State of Hawaii and the U.S. territories in the Pacific Ocean that could be impacted by a potential tsunami," FEMA administrator Craig Fugate said.
"FEMA stands ready to assist should a request for assistance be made, and does have pre-deployed assets in Hawaii, including food, water, generators and other resources," he said. "We urge all individuals to follow the direction provided by local officials."
The U.S. has also offered to help Chile.
In his statement Saturday, Obama said
U.S. urban search and rescue teams from Los Angeles and Virginia are on standby and could be in Chile in a matter of hours," State Department spokesman P.J. Crowley told Fox News. But no request has been made yet for those units, he said.
The White House "is closely monitoring the situation, including the potential for a tsunami," White House spokesman Robert Gibbs said.
"Our thoughts a prayers are with the people of Chile and we stand ready to help in this hour of need," he said.
The U.S. Embassy is reaching out to the Chilean government to offer assistance, Crowley told Fox News, noting that the country benefits from sound infrastructure.
"It's a powerful earthquake but Chile has a lot of experience with earthquakes and from devastating earthquakes in 1960," Crowley said.
"Chile has both significant capabilities and strong building codes," he said.
Crowley added that the more than 100 personnel at the embassy in the capital of Santiago are "all OK and have been accounted for."
Large waves caused by the earthquake already have hit the coast of Chile, and a tsunami warning has been issued for Hawaii and across the Pacific. Officials in Hawaii are gearing up to evacuate coastal areas Saturday morning as the state braces for a wave surge in the afternoon.
In Chile, dozens have been reported dead so far, though the extent of the devastation from the magnitude 8.8 quake isn't yet known.
State Department spokeswoman Megan Mattson said the U.S. sends its "heartfelt condolences and prayers for the residents of Chile." She has no immediate information about the welfare of Americans visiting or living in the country.
The State Department advises Americans seeking information on family and friends in Chile to contact the Bureau of Consular Affairs at 1-888-407-4747.
Meanwhile, the Pentagon says it's had no reports that the big earthquake in Chile has affected the U.S. military in the region -- on land or at sea.
The Defense Department also says it is in close contact with the U.S. Embassy in Chile, has personnel on the ground in Santiago but hasn't been asked yet to provide assistance.
In cases of natural disasters, the affected country usually accepts an offer of help made by the State Department.
The State Department can request that Defense Department personnel help carry out that mission.
U.S. Southern Command, or Southcom, based in Miami, oversees the U.S. military presence in South America.
Southcom spokesman Jose Ruiz told Fox News that it is still "very, very early" after the quake and Chile is a developed nation. So the Chilean government will need to determine whatever needs it may have, and Southcom would be happy to help if asked, the spokesman said.
Fox News' Mike Emanuel, Justin Fishel and The Associated Press contributed to this report.