Doan Huu Gia, the chief of search and rescue coordination centre, said that six planes and seven ships from Vietnam were searching for the object but nothing had so far been found.
Test results from an oil slick found in the same area are due back on Monday afternoon and should show if the oil comes from the plane or not.
Azharuddin Abdul Rahman, director general of Malaysian civil aviation department, told a press conference on Monday morning that hijacking had not been ruled out as a cause of the plane’s disappearance and that investigators were looking at “all angles”.
He said there was still no sign of the missing plane and that no signal from its black box had been detected.
Meanwhile, questions remain over how two passengers managed to board the aircraft using stolen passports.
Interpol confirmed it knew about the stolen passports but said no authorities checked its vast databases on stolen documents before the Boeing jetliner departed on Saturday from Kuala Lumpur en route to Beijing with 239 people on board.
Warning “only a handful of countries” routinely make such checks, Interpol secretary general Ronald Noble chided authorities for “waiting for a tragedy to put prudent security measures in place at borders and boarding gates.”
More than two days after Malaysia Airlines Flight MH370 went missing, the final minutes before its disappearance remained a mystery. The plane lost contact with ground controllers somewhere between Malaysia and Vietnam.
The jetliner apparently fell from the sky at cruising altitude in fine weather, and the pilots were either unable or had no time to send a distress signal - unusual circumstances under which a modern jetliner operated by a professional airline would crash.
Authorities were checking on the identities of the two passengers who boarded the plane with stolen passports. On Saturday, the foreign ministries in Italy and Austria said the names of two citizens listed on the flight’s manifest matched the names on two passports reported stolen in Thailand.
"I can confirm that we have the visuals of these two people on CCTV," Malaysian Transport Minister Hishammuddin Hussein said at a news conference late Sunday, adding that the footage was being examined. "We have intelligence agencies, both local and international, on board."
The thefts of the two passports - one belonging to Austrian Christian Kozel and the other to Luigi Maraldi of Italy - were entered into Interpol’s database after they were stolen in Thailand in 2012 and last year, the police body said.
Electronic booking records show that one-way tickets with those names were issued on Thursday from a travel agency in the beach resort of Pattaya in eastern Thailand. A person who answered the phone at the agency said she could not comment.
But no authorities in Malaysia or elsewhere checked the passports against the database of 40 million stolen or lost travel documents before the Malaysian Airlines plane took off.
In a forceful statement, the Interpol chief said he hoped “that governments and airlines worldwide will learn from the tragedy.”
"Now, we have a real case where the world is speculating whether the stolen passport holders were terrorists," Mr Noble said. "Interpol is asking why only a handful of countries worldwide are taking care to make sure that persons possessing stolen passports are not boarding international flights."
Details also emerged on Sunday about the itineraries of the two passengers traveling on the stolen passports.
A telephone operator on a China-based KLM hotline confirmed on Sunday that passengers named Maraldi and Kozel had been booked on one-way tickets on the same KLM flight, flying from Beijing to Amsterdam on Saturday. Maraldi was to fly on to Copenhagen, Denmark, and Kozel to Frankfurt, Germany.
She said the pair booked the tickets through China Southern Airlines, but she had no information on where they bought them.
As holders of EU passports with onward flights to Europe, the passengers would not have needed visas for China.
Interpol said it and national investigators were working to determine the true identities of those who used the stolen passports to board the flight. White House Deputy National Security Adviser Tony Blinken said the US was looking into the stolen passports, but that investigators had reached no conclusions.
Of the 227 passengers and 12 crew members on board, two-thirds were Chinese, while the rest were from elsewhere in Asia, Europe and North America, including three Americans.
Family members of Philip Wood, a 50-year-old IBM executive who was on board the plane, said they saw him a week ago when he visited them in Texas after relocating to Kuala Lumpur from Beijing, where he had worked for two years.
"There is a shock, a very surreal moment in your life," said Wood’s brother, James Wood.
The other two Americans were identified on the passenger manifest as 4-year-old Nicole Meng and 2-year-old Yan Zhang. It was not known with whom they were traveling.
After more than 30 hours without contact with the aircraft, Malaysia Airlines told family members they should “prepare themselves for the worst,” Hugh Dunleavy, the commercial director for the airline, told reporters.
Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over a large area. If the plane enters the water before breaking up, there can be relatively little debris.
Edited by Bonnie Malkin