Planes searching an area where Chinese satellites spotted possible debris from a missing Malaysian passenger jet have found no sign of wreckage, officials said Thursday, dimming hopes of a breakthrough in the mystery. China said late Wednesday its satellites had detected three large floating objects in a suspected crash site near where the Malaysia Airlines plane, which disappeared Saturday on a flight from Kuala Lumpur to Beijing with 239 people on board, lost contact. But Vietnam said Thursday that two of its planes dispatched to the area had found no trace of the airliner. Reeling from a storm of criticism about its handling of the crisis, Malaysia also sent an aircraft to investigate the reported sighting in the South China Sea, pledging to pursue all "concrete clues" -- but that it had also found nothing as of Thursday afternoon. "Nil sighting," the Malaysian air force's director-general of operations, Affendi Buang, told AFP. The search for Malaysia Airlines flight MH370 -- which entered a sixth day Thursday -- has been blighted by false alarms, swirling rumours and contradictory statements about its fate. "Every day it just seems like it's an eternity," Danica Weeks, whose husband Paul was on board, told CNN from their home in the Australian city of Perth. Fighting back tears, she described how Paul had left his wedding ring and watch with her for safekeeping before starting his journey to a mining venture in Mongolia. "I'm praying that I can give (them) back to him. It's all I can hold onto. Because there's no finality to it and we're not getting any information," she said. China's state science and technology administration said a satellite had captured images of the objects in a suspected crash area on Sunday, and the information was being analysed. China will keep up the search "as long as there is a glimmer of hope", Premier Li Keqiang said. The passengers included 153 Chinese citizens, and Li told his once-a-year press conference: "Those people's families and friends are burning with anxiety." It was not immediately clear why the satellite information has only just come to light. The region is criss-crossed by busy shipping lanes and littered with debris, complicating the search. Large oil slicks found by Vietnamese planes on Saturday yielded no trace of the Boeing 777, while previous sightings of possible wreckage proved to be false leads. The search for the plane now encompasses both sides of peninsular Malaysia, over an area of nearly 27,000 nautical miles (more than 90,000 square kilometres) -- roughly the size of Portugal -- and involves the navies and air forces of multiple nations. Theories about the possible cause of the disappearance range from a catastrophic technical failure to a mid-air explosion, hijacking, rogue missile strike and even pilot suicide. - 'This could be it' - The objects detected by the Chinese satellite were seen roughly 200 kilometres (124 miles) east of the location of the plane's last reported contact roughly mid-way between the coasts of Malaysia and Vietnam. "That would make sense if the debris were there," said Gerry Soejatman, a Jakarta-based independent aviation analyst. "It is very possible that this could be it. The satellite image is what is seen at the time the debris would have drifted and/or sunk by then. It can be calculated to find where it is now." The objects were spread across an area on the eastern-most margin of the original search zone, with a radius of 20 kilometres (12 miles). The new information prompted the focus of the search to swing back to the original flight path, after a shift in recent days to Malaysia's west coast -- far from the last known location. "We will look at all areas especially the ones with concrete clues," a spokesman for Malaysia's civil aviation department said after the Chinese announcement. But Vietnam Civil Aviation Authority deputy director Dinh Viet Thang told AFP that two aircraft sent to inspect areas near where the suspicious objects were detected by China "have returned and we found nothing so far". Earlier, he said Vietnamese officials had only seen the report of the sighting on the Internet, raising fresh questions about the coordination of the huge search. And the US Navy, which is contributing two destroyers and two surveillance planes to the vast search, appeared to be treating the latest news with caution. "I do not have specific information about that satellite image," Commander William Marks of the USS Blue Ridge, the command ship of the US Seventh Fleet, told CNN. - 'Good-quality images' - The China Centre for Resources Satellite Data and Application said in a statement on its website earlier this week that it had deployed eight land observation satellites to scour the suspected crash area. By Tuesday morning, it had obtained images covering 120,000 square kilometres, describing their quality as "rather good". China has also requested assistance from a fleet of Earth-monitoring satellites under an international charter designed to aid emergency efforts. US authorities said their spy satellites had detected no sign of a mid-air explosion. In a new twist, Malaysian police said Thursday they were investigating the two pilots, after an Australian television report of a past cockpit security breach. Malaysia Airlines has said it was "shocked" over allegations that First Officer Fariq Abdul Hamid, 27, along with a fellow pilot, violated airline rules in 2011 by allowing two young South African women into their cockpit during a flight. It also emerged that months before the Malaysia Airlines jet vanished, US regulators had warned of a "cracking and corrosion" problem on Boeing 777s that could lead to a drastic drop in cabin pressure and possible mid-air break-up. On Thursday, Malaysia Airlines said it would retire the flight codes MH370 and MH371 as a mark of respect. burs/dr/st/sls