An investigation into the disappearance of a Malaysia Airlines jetliner is focusing more on a suspicion of foul play, as evidence suggests it was diverted hundreds of kilometres off course, sources familiar with the Malaysian probe said.

In a far more detailed description of military radar plotting than has been publicly revealed, two sources told Reuters an unidentified aircraft that investigators suspect was missing Flight MH370 appeared to be following a commonly used navigational route when it was last spotted early on Saturday, northwest of Malaysia.

That course — headed into the Andaman Sea and toward the Bay of Bengal in the Indian Ocean — could only have been set deliberately, either by flying the Boeing 777-200ER jet manually or by programming the auto-pilot.

A third investigative source said inquiries were focusing more on the theory that someone who knew how to fly a plane deliberately diverted the flight hundreds of miles off its scheduled course from Kuala Lumpur to Beijing.

A family member of a passenger onboard the missing Malaysia Airlines Flight MH370 sobs during a briefing from the Malaysia Airlines at a hotel in Beijing on March 14. (Kim Kyung-Hoon/Reuters)

"What we can say is we are looking at sabotage, with hijack still on the cards," said the source, a senior Malaysian police official.

One of the most baffling mysteries in the history of modern aviation remains unsolved after nearly a week.

The latest radar evidence is consistent with the expansion of the search for the aircraft to the west of Malaysia, possibly as far as the Indian Ocean.

There has been no trace of the plane nor any sign of wreckage as the navies and military aircraft of more than a dozen countries scour the seas across Southeast Asia.

Vastness of Indian Ocean 'biggest challenge'

Malaysian Transport Minister Hishammuddin Hussein said he could not confirm the last heading of the plane or if investigators were focusing on sabotage.

"A normal investigation becomes narrower with time ... as new information focuses the search, but this is not a normal investigation," he told a news conference. "In this case, the information has forced us to look further and further afield."

Muslims attend Friday prayers at the National Mosque in Kuala Lumpur during a special prayer session for the passengers of the missing Malaysia Airlines Flight MH370. (Damir Sagolj/Reuters)

Investigators were still looking at "four or five" possibilities, including a diversion that was intentional or under duress, or an explosion, he said. Police would search the pilot's home if necessary and were still investigating all 239 passengers and crew on the plane, he said.

If the jetliner did stray into the Indian Ocean, a vast expanse with depths of more than 7,000 metres, the task faced by searchers would become dramatically more difficult. Winds and currents could shift any surface debris tens of nautical miles within hours, dramatically widening the search area with each passing day.

"Ships alone are not going to get you that coverage, helicopters are barely going to make a dent in it and only a few countries fly P-3s [long-range search aircraft]," William Marks, spokesman for the U.S. Seventh Fleet, told Reuters.

"So, this massive expanse of water space will be the biggest challenge."

U.S. destroyer heading to Strait of Malacca

The U.S. Navy was sending an advanced P-8A Poseidon plane to help search the Strait of Malacca, a busy sea lane separating the Malay Peninsula from the Indonesian island of Sumatra. It had already deployed a Navy P-3 Orion aircraft to those waters.

U.S. defence officials told Reuters that the U.S. Navy guided-missile destroyer USS Kidd was heading to the Strait of Malacca, answering a request from the Malaysian government. The Kidd had been searching the areas south of the Gulf of Thailand, along with the destroyer USS Pinckney.

"It's my understanding that based on some new information that's not necessarily conclusive — but new information — an additional search area may be opened in the Indian Ocean," White House spokesman Jay Carney told reporters in Washington.

Carney did not specify the nature of the new information.

Satellites picked up faint electronic pulses from the aircraft after it went missing on Saturday, but these gave no immediate information about where the jet was heading and little else about its fate, two sources close to the investigation said on Thursday.

The signals were part of the routine pings that are sent out through the plane's Aircraft Communications Addressing and Reporting System (ACARS), transmitting diagnostic data about the performance of engines and other equipment to engine manufacturers and airlines.

U.S. experts are still examining the data to see if any information about its last location could be extracted, a source close to the investigation told Reuters. Malaysia's civil aviation chief confirmed on Friday the government was working with U.S. investigators to establish if there was any satellite information that could help locate the airliner.

Last radar sighting

The last sighting of the aircraft on civilian radar screens came shortly before 1:30 a.m. on Saturday, less than an hour after take-off. It was flying as scheduled across the mouth of the Gulf of Thailand on the eastern side of peninsular Malaysia, heading toward Beijing.

However, Malaysia's air force chief said on Wednesday that an aircraft that could have been the missing plane was plotted on military radar at 2:15 a.m., 320 kilometres northwest of Penang Island off Malaysia's west coast.

This position marks the limit of Malaysia's military radar in that part of the country, a fourth source familiar with the investigation told Reuters.

Sources: CBC News stories, Reuters, AP (CBC)

Malaysia says it has asked neighbouring countries for their radar data but has not confirmed receiving the information. Indonesian and Thai authorities said on Friday they had not received an official request for such data from Malaysia.

The fact that the plane — if it was MH370 — had lost contact with air traffic control and was invisible to civilian radar suggested someone on board had turned off its communication systems, the first two sources said.

They also gave new details on the direction in which the unidentified aircraft spotted on the military's radar was heading — saying it was following aviation corridors identified on maps used by pilots as N571 and P628, routes used by commercial planes flying from Southeast Asia to the Middle East or Europe.

Hishammuddin said it remained unclear if that aircraft was MH370. "We need to get verification, and we are working very closely with the experts," he said.

Haze affecting search

Ships and aircraft are now combing a vast search area that had already been widened to cover both sides of the Malay Peninsula and the Andaman Sea.

An already difficult search task has been complicated in some areas by a choking haze caused by burning forest and farmland that has enveloped much of Malaysia and spilled into the Strait of Malacca. The haze, exacerbated by a prolonged dry spell, has reached hazardous levels in several spots.

"The haze will affect the search and rescue operations, for sure," Amirzudi Hashim, a senior meteorologist at the National Weather Centre, told Reuters. "The visibility at the ground level has dropped to less than three kilometres."

India had deployed ships, planes and helicopters from the remote Andaman and Nicobar Islands, at the juncture of the Bay of Bengal and the Andaman Sea, an Indian military spokesman, Harmeet Singh, said on Friday.