Ten days after the deadly sinking of a South Korean ferry that is turning out to be one of the country' worst maritime disasters, Seoul on Friday mobilized all available resources to search for the more than 100 people still missing.

The government emergency task force said it has dispatched the largest number of Coast Guard, Navy and civilian divers and state-of-the art rescue equipment to search for bodies inside the 6,825-ton Sewol that sank in southwestern waters on April 16.

At least 181 passengers have been confirmed dead, with some 120 others still missing. The missing, most of whom were high school students on a school trip, are believed to be trapped inside the fourth floor of the upturned vessel in waters off Jindo Island.

Rescuers have not found a single survivor since 174 people were rescued on the day of the accident. Among the rescued are the ferry's captain and most of its crew who were later arrested for allegedly abandoning passengers.

The USNS Safeguard, the U.S 7th Fleet diving and recovery vessel, is currently heading toward South Korea's southwest coast and is expected to arrive at the scene later on Friday, Coast Guard officials said.

The Coast Guard said it will deploy the vast number of civilian divers into the operation, including Lee Jong-in, the head of Alpha Sea Rescue Co. and an expert in underwater rescue, to use his diving bell.

Lee, who had previously participated in major maritime disasters such as the sinking of the warship Cheonan in 2010, is an expert in using the diving bell, the airtight chamber used for transporting divers underwater. The equipment enables the divers to stay underwater longer.

The government also said that Lee Ju-young, the minister of oceans and fisheries, and Kim Suk-kyoon, the chief of the Coast Guard, will stay at a nearby port to command the search and rescue operation.

Obama lands in Seoul as Pyongyang mulling new nuclear test

US President Barack Obama arrived in Seoul Friday to growing signs the Democratic People's Republic of Korea (DPRK) was defiantly readying another nuclear test, and with South Koreans in a state of national mourning over the sinking of a ferry full of schoolchildren.

The American president landed at a US Air Force base outside a South Korean capital still wracked with grief over the 300 people dead or missing more than a week after the disaster.

Obama is expected to offer personal condolences to his counterpart Park Geun-Hye over the tragedy, but Seoul's unpredictable northern neighbor is set to dominate the agenda.

Satellite photos taken just two days ago showed additional activity at the DPRK's Punggye-ri test site that is "probably related to preparations for a detonation", the US-Korea Institute at Johns Hopkins University said.

The report echoed recent warnings from South Korea that the DPRK might be planning a test to coincide with Obama's two-day visit.

If Pyongyang presses ahead with its fourth nuclear test it would be a clear challenge to Obama's bid to cement America's role as a Pacific power.

His four-nation Asian tour began in Tokyo, where he urged China to rein in the wayward Pyongyang, saying Beijing had a "critically important" role to play in defusing tensions on the volatile peninsula.

"North Korea (the DPRK) has engaged in provocative actions for the last several decades," he said.

"It's been an irresponsible actor on the international stage for the last several decades. They are the most isolated country in the world. They are subject to more international sanctions and international condemnation than any country in the world."

In an interview ahead of his arrival in Seoul, he warned the DPRK could expect a "firm response" if it made "the mistake" of conducting another nuclear test.

Pyongyang, for its part, slammed Obama's trip earlier this week as a "dangerous" move that would escalate military tension and bring the "dark clouds of a nuclear arms race" over the Korean peninsula.

Adding to the tense mix was the news that a South Korean naval vessel had fired warning shots after two DPRK patrol boats crossed the disputed maritime border Friday. The boats quickly retreated.

Death toll rises to 181 in S.Korean ferry sinking disaster

Death toll from a sunken South Korean ferry continued to rise to 181 on Friday as divers kept searching bodies, or possible survivors, inside the submerged vessel.

On the tenth day since the ferry carrying 476 people capsized off the country's southwestern coast on April 16, seven more bodies were recovered from the ship, raising the death toll to 181, with 121 still missing. The number of the rescued has been unchanged at 174 since the first day of the deadly incident.

The government-wide disaster response headquarters said at a press briefing that 88 divers will search passenger cabins on the third and fourth floors of the five-story vessel from 5 a.m. to 5 p.m. local time. Only 10 divers can trace guideline ropes into the submerged vessel at a time because 5 ropes are being snaked into the ship.

On the previous day, 81 divers hunted for bodies at the middle part of the third floor and cabins of the fourth floor on the stern side. As of 10 a.m.Friday, 86 male and 95 female passengers have been confirmed dead, the headquarters.

The U.S. Navy's salvage ship USS Safeguard was scheduled to arrive in waters near Jindo Island at around 3 a.m. Saturday to help in rescue efforts. U.S. Navy divers will join the search operations, if needed.

Rescue experts from the United States, the Netherlands, Britain and Japan have been providing counseling on the rescue operations, the headquarters said.

Rescue operations were hampered Thursday as tidal currents became faster than expected. Weather forecast had said the currents in the area would slow down for four days through Thursday. It was forecast to rain from Saturday night.

Only 15 bodies were retrieved Thursday, much lower than 38 bodies discovered Wednesday. Divers recovered 36 people Tuesday and 28 others Monday respectively.

Hundreds of divers resumed entering the ship at around 5 a.m. local time to search passenger cabins on the third and fourth floors of the five-story vessel where many passengers, two thirds of whom were high school students, have been believed to be trapped.

The 476 passengers included 325 Danwon High School students and 14 teacher. With hopes fading for possible survivors as the death toll continued to grow, the high school in Ansan, a city south of Seoul, established an altar in a gymnasium to mourn the young students dead in the incident.