A fleet of Earth-monitoring satellites has joined the search for missing Malaysia Airlines flight MH370, the organisers of the satellite pool said on Wednesday.

China on Tuesday requested activation of the so-called International Charter on Space and Major Disasters, the organisation said on its website.

Under the agreement, 15 space agencies or national space institutes help emergency or relief efforts by passing on images from satellites flying over the location.

"Satellite imagery is now being employed to search for any evidence of the plane, both before and after it disappeared," the website said.

"As of 12 March 2014, the search remains ongoing."

The activation request came from the China Meteorological Administration, it said.

"They are providing high-resolution data," a European Space Agency (ESA) official, whose organisation is a charter member, told AFP.

He was unable to say which geographical areas were being covered.

The Charter, which took effect in 2000, has been activated more than 400 times, but this is the first time it has been called in to help the search for a missing aircraft, according to records of its operations.

It has been mostly used in the aftermath of earthquakes and floods when rescue teams needed to identify badly damaged zones or roads, railways and bridges that are still passable.

It was invoked last November to aid swathes of the Philippines ravaged by Typhoon Haiyan.

Its biggest operation was three years ago, following the earthquake and tsunami that struck northeastern Japan, when a hundred maps were drawn up from its data.

Once activated, space technicians determine which satellites are available and best suited for the task of providing ground radar data or photos.

They then send a request to the satellite's operator, which programmes the orbiter to take pictures as it flies a zone. The data are usually available within 24 hours and are provided for free.

US satellite firm DigitalGlobe has set up a crowdsourcing platform, inviting Internet users to comb through images to search for clues of the missing aircraft.

The search for MH370: key areas of confusion
Kuala Lumpur (AFP) March 12, 2014 - False alarms, swirling rumours and contradictory statements have made the wait all the more agonising for the families of the 239 people on board the missing Malaysia Airlines flight MH370.

As the search dragged into its fifth day, here are some of the key areas of confusion:

Did the plane veer off course?

Malaysia's air force chief on Sunday raised the possibility that the plane inexplicably turned back after taking off from Kuala Lumpur en route to Beijing a day earlier.

General Rodzali Daud said the theory was "corroborated by civil radar", without giving further details.

Late Tuesday, Rodzali was quoted by a Malay-language paper as saying the jet had been tracked hundreds of miles from its intended flight path, over the Strait of Malacca off western Malaysia.

Vietnam had initially said the plane was approaching its airspace when it vanished from radar screens.

Rodzali has since insisted that he did not make the comments attributed to him by the Berita Harian newspaper, and the report was "inaccurate and incorrect".

The search on Wednesday swung even further up Malaysia's west coast, towards the Andaman Sea, but officials gave no indication there was a firm reason to expand the search other than its failure to bear fruit so far.

In his latest remarks, Rodzali said authorities were investigating an unidentified flying object about 200 miles (320 kilometres) northwest of the Malaysian state of Penang -- a long way from the flight path -- around the time the plane vanished.

"We are corroborating this. We are not saying this is MH370. It's an unidentified plot," he said at a news conference Wednesday.

Timings

Officials say contact with the aircraft was lost at around 1:30 am Malaysian time (1730 GMT Friday), about an hour after take-off from Kuala Lumpur International Airport. Initially, Malaysia Airlines had put the last contact time at 2:40 am.

The timing of 1:30 am would place the plane between Malaysia and Vietnam, where Vietnamese air traffic control and flight-tracking websites say the plane vanished off radar.

The later time of 2:40 am could suggest the plane had indeed veered radically off-course. That theory has gained credence given the expanded search area off Malaysia's west coast.

And at Wednesday's news conference , the air force chief said the "unidentified plot" was spotted on radar northwest of Penang at 2:15 am.

Search areas

The search began in the South China Sea and Gulf of Thailand, on the approach to Vietnamese airspace.

Since then, Malaysian authorities have widened the search radius several times as well as shifting its focus, fuelling accusations of official bungling and a slow-footed response.

Authorities have not said whether they have any firm indications that the plane might be in the Andaman Sea.

Vietnam initially suspended its air and sea search Wednesday as it waited for Malaysia to clarify the new direction of the hunt, but later sent two aircraft and nine boats back out.

In total, 42 ships and 39 aircraft are taking part in the multinational search including from China and the United States.

Debris

There have been several false alarms linked to debris spotted in busy shipping lanes in Southeast Asian waters.

Large oil slicks found by Vietnamese planes on Saturday yielded no trace of the plane; nor did debris found Sunday near Tho Chu island, part of a small archipelago off southwest Vietnam.

Malaysia sent ships to investigate a sighting of a possible life raft on Monday, but a Vietnamese vessel that got there first found only flotsam.

Chemical analysis by Malaysia on Monday found no link between oil found at sea and the missing plane.

Stolen passports

Revelations that two of the passengers were travelling on stolen EU passports fuelled early speculation that the plane was the victim of a terrorist attack.

Malaysia's national news agency Bernama on Sunday quoted Home Minister Zahid Hamidi as saying the two suspect passengers had "Asian features", without elaborating.

It emerged on Tuesday that the pair appear to be Iranian illegal immigrants who were seeking a new life in Europe.

Phones ringing

Chinese media have reported that relatives have heard ringing tones when trying to call their missing loved ones' mobile phones.

The accounts of some passengers on Chinese messaging tool QQ show they had been online, other reports say, although the operator says that failure to shut the software down properly can give that impression.

Alfred Siew, a Singapore-based technology commentator, admits it is a "mystery", but said the matter could be merely due to a network error affecting some phones.

Luggage

Azharuddin Abdul Rahman, head of Malaysia's Department of Civil Aviation, said Monday that five passengers who had purchased tickets and checked baggage did not make the flight.

He told journalists their luggage was removed from the plane, per standard procedure, when routine checks indicated the five passengers had not boarded before take-off.

But Malaysia's national police chief Khalid Abu Bakhar insisted Tuesday that all passengers who booked the flight did board in the end.

However, muddying things further, Malaysia Airlines issued a statement hours later saying there were indeed four passengers who had valid bookings but did not check-in for the flight. They were replaced by four stand-by passengers, the airline's chief executive said Wednesday.

Balotelli

Malaysia's civil aviation chief, Azharuddin, also drew scorn on social media by referring to black Italian footballer Mario Balotelli when discussing the two suspicious passengers who boarded the plane with stolen passports.

When asked what the two suspects travelling on EU passports looked like, Azharuddin referenced Balotelli, who was born in Italy to Ghanaian parents and is an Italian international player, as an example of how one's skin colour does not necessarily indicate nationality.

Malaysia's transport ministry later issued a statement saying "no ill feelings" were meant by the comment, but the social media reaction underlined feelings of embarrassment with so much world attention focussed on the plane search.