Concepcion, Chile (CNN) -- Two strong aftershocks shook parts of Chile on Friday morning, hours before U.N. Secretary-General Ban Ki-moon was to meet with Chilean President Michele Bachelet and her successor.
An aftershock off the coast of the Bio Bio region earlier Friday had a magnitude of 6.0, the survey said. It was originally reported as having a magnitude of 6.3.
The 6.6-magnitude aftershock struck about 20 miles (35 kilometers) from Concepcion, which is near the epicenter of Saturday's devastating 8.8-magnitude quake.
The country is recovering from that massive quake -- the fifth strongest in about 100 years -- that toppled buildings, triggered a tsunami and killed several hundred people. Thousands were left homeless.
The Tsunami Center at the National Oceanic and Atmospheric Administration did not issue a tsunami warning following the 6.6 aftershock.
Strong waves might be felt out at sea, but a 6.6 temblor probably would not generate a tsunami, USGS geophysicist Jessica Sigala said.
More than 150 aftershocks of 5.0 or greater have hit Chile since Saturday. The largest aftershock -- a 6.9 -- rattled the South American nation about an hour and a half after the initial 8.8-magnitude quake, according to the USGS.
The situation remains critical in the hardest-hit areas as the strong aftershocks continue to shake the country.
Food, water and restoration of basic services such as electricity are top priorities, said the U.N. Office for the Coordination of Human Affairs.
The Chilean government, which is leading the rescue and relief efforts, has asked the United Nations for items such as field hospitals with surgical facilities, dialysis centers, generators, satellite phones, structural damage evaluation systems, saltwater purifying systems, mobile bridges and field kitchens.
Meanwhile, a 24-hour telethon, called "Chile Helps Chile," is slated for Friday. Organizers aim to raise $27 million to help quake victims.
Ban will meet with Bachelet and Sebastian Piñera, who takes office Thursday and has named six officials to lead recovery efforts.
"Our government will not be a government of the earthquake," Piñera said. "Our government will be a government of reconstruction."
Ban also plans to visit Concepcion to see the devastation.
Bachelet has dispatched more than 13,000 soldiers and other military personnel to restore order in the area. Food and water started to arrive late Tuesday in Concepcion, and officials said they distributed 3,500 aid packages Wednesday. But many residents have complained that federal aid has been slow to arrive.
"Today, we have not received one measure of water from the National Emergency Office, understanding that there are some difficulties," said Eduardo Soto, mayor of Rancagua in central Chile.
Bachelet toured parts of central and southern Chile on Thursday. She first visited Talcahuano, where a quake-induced tsunami roared through many parts of the seaport. Bachelet also toured Talca, another devastated area.
"This is the time for action," she said later Thursday on returning to the capital, Santiago, where she met with leaders of the Catholic Church.
The church has been a partner in providing help in the quake's aftermath. Monsignor Alejandro Goic shared a note from Pope Benedict XVI, expressing sadness about the tragedy and supporting the Chilean people.
The top priority remained the needs of survivors, followed by reconstruction efforts, Bachelet said.
Chile has announced three days of national mourning starting Sunday. Every house has been authorized to hang the national flag in memory of those who perished.
The death toll was revised downward Thursday as authorities reviewed discrepancies in the reported number of dead in the Maule region.
The casualty count in Maule, originally reported at 587, included more than 200 people who are missing but not confirmed dead, said Deputy Interior Minister Patricio Rosende. Therefore, the official nationwide death toll is lower than the 802 figure that emergency authorities gave.
To limit confusion, Rosende read aloud the names of 279 Chileans who had been killed and identified by Thursday evening. He said would update that official number regularly. The new tally does not account for hundreds of unidentified victims.
With disasters of this magnitude, "the number of dead change daily," Rosende said. "It takes months sometimes to compile the information, because one of the biggest problems in the affected areas is the lack of precision and uncertainty at the scene."
In other developments, the Chilean Davis Cup tennis team will open competition in Coquimbo, Chile, against Israel on Saturday instead of Friday. Team officials and players said they would be playing in honor of quake victims.
"It will be difficult, but we will do our best for our country," said player Fernando Gonzalez.