BEIJING — Vietnamese aircraft searching for a missing Malaysia Airlines jet spotted what authorities said may be a door from the plane and boats were attempting to reach the debris early Monday.

As about 40 ships and more than 20 airplanes continued looking for Flight MH370 for a third day, Interpol confirmed that at least two stolen passports were used by passengers on the plane, and the police agency's head railed that few countries were regularly checking its international database on lost and stolen travel documents.

Interpol said the two stolen passports — one Italian, one Austrian — had been entered into the agency's database after they had been stolen in Thailand in 2012 and 2013 but no checks of those passport numbers were made by any country between the time they were entered into Interpol's database and the time that the Malaysia Airlines flight departed Kuala Lumpur early Saturday, bound for Beijing.

"Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol's databases," the agency's secretary-general, Ronald K. Noble, said in a statement.

"What is important at the moment is to find out what caused Malaysian [Airlines] Flight 370 to go missing, and in this regard Interpol is making all needed resources available to help relevant authorities in Malaysia and elsewhere find out what happened," he said, adding that the agency was in contact with officials in Malaysia and elsewhere to "determine the true identities of the passengers who used these stolen passports."

He also said the agency would review the documents used by other passengers to see whether others were traveling on lost or stolen papers.

Vietnam's state-run Thanh Nien newspaper said two ships from the country's maritime police were en route to the area where the object that looked like a door was spotted, about 60 miles south of Tho Chu island off the nation's south coast, Associated Press reported.

"From this object, hopefully [we] will find the missing plane," the deputy chief of staff of Vietnam's army, Lt. Gen. Vo Van Tuan, told the paper.

If the Boeing 777 disintegrated in the air, the debris could be spread over a massive area hundreds of miles across. Malaysian authorities said Sunday that a patrol ship from their country's Maritime Enforcement Agency discovered a large oil slick in the waters 100 nautical miles from Tok Bali that could be connected to the plane.

The agency's director-general, Mohamed Amdan Kurish, said the ship was ordered to collect samples of the oil to determine whether it came from the plane, which vanished Saturday en route to the Chinese capital with 239 people aboard.

If the plane did crash into the sea, "obviously we will find clothes, bags and debris that float," he added, Malaysia's state-run Bernama news agency reported. Fishermen working in the area were also going to be interviewed by the agency as part of the search operation.

Indonesia, Thailand, Australia, China, the U.S. and other countries were participating in the quest to find the jetliner off southern Vietnam where the Gulf of Thailand meets the South China Sea. Malaysia was deploying submarine rescue vessels, officials said.

With no wreckage yet to examine, investigators were probing the identities of the passengers who used the stolen passports. Malaysia's acting transportation minister, Hishammuddin Hussein, said at one point Sunday that authorities were looking at four passengers "said to have been traveling on fake passports." But he later backtracked and said investigators were looking at only two suspected cases of stolen identity, creating some confusion.

Azharuddin Abdul Rahman, Malaysia's director-general of civil aviation, said at a news conference in Malaysia that investigators had closed-circuit TV images of the two men traveling on the European passports. But when journalists pressed him to describe the men physically, he declined.

"Our main concern is to find the aircraft," he said.

China's ambassador to Malaysia, Huang Huikang, said the embassy was investigating whether the two men had proper documents to go to Beijing, state-run CCTV reported.

Hugh Dunleavy, head of Malaysia Airlines' commercial operations, told reporters in Beijing: "As far as we are aware, every one of these people onboard that aircraft had a visa to go to China. Which means those passports were in possession of the Chinese Embassy before those visas were issued."

However, Beijing has a policy whereby some passengers in transit to third countries can stay in the Chinese capital for 72 hours without a visa, so the two men may not have needed to apply for Chinese visas if they had tickets to another destination.

London's Telegraph newspaper said it had confirmation from two ticketing agents for China Southern Airlines — which had a code-share arrangement with Malaysia Airlines for the flight — that the two men were booked to fly on to Amsterdam on the KLM airline Saturday morning. The paper said the two tickets were numbered consecutively, suggesting that they were bought at the same time.

A travel agent in Beijing, who asked not to be identified, said airlines "have no legal means or responsibility to verify the accuracy of passport information," and that it's up to airport and immigration authorities to check documents.