Relatives gather at Surabaya, Changi airports as search operations continue

Around 11 hours after it disappeared, Indonesian air force jets had yet to find any signs of the Airbus A320-200, with dusk fast approaching and fuel running low.

Air traffic controllers lost contact with the twin-engine aircraft around an hour after it left Juanda international airport at Surabaya in east Java at 5:20am (2220 GMT Saturday).

Shortly before disappearing, AirAsia said the plane had asked permission from Jakarta air traffic control to deviate from its flight plan and climb above bad weather in an area noted for severe thunderstorms.

The airline said 156 of those on board Flight QZ8501 were Indonesians, with three South Koreans and one person each from Singapore, Malaysia and France.

Britain later said one of its citizens was on board the jet. It was not yet confirmed whether they had dual nationality.

There were 138 adult passengers, 16 children and an infant, in addition to five cabin crew and the pilot and co-pilot, who is believed to be French.

Two Indonesia air force planes and a helicopter were searching the waters around the islands of Bangka and Belitung in the Java Sea, near Kalimantan island.

"We have not been able to visually detect any signs," air force spokesman Hadi Cahyanto told AFP, adding search boats were still on their way to the area, around halfway along the missing aircraft's expected flight path.

"The weather is quite good. However, we only have a few hours more to go as our fuel will run out. By then it will also get dark... the planes will have to return to Jakarta."

A Singaporean C-130 military transport aircraft was also deployed, after Indonesia accepted help from its Southeast Asian neighbour, while Malaysia said it had committed "military assets" to the search.

The aircraft was operated by AirAsia Indonesia, a unit of Malaysian-based AirAsia which dominates Southeast Asia's booming low-cost airline market.

AirAsia's flamboyant boss Tony Fernandes, a former record industry executive who acquired the then-failing airline in 2001, said he was on his way to Surabaya, where most of the passengers are from.

"My only thought(s) are with the passengers and my crew," he added on his Twitter page.

With hard details few and far between, panicked relatives gathered at Singapore's Changi airport.

In Surabaya hundreds of Indonesians descended on the terminal, hoping for news of the missing jet.

A 45-year-old woman told AFP that she had six family members on the plane.

"They were going to Singapore for a holiday," she said.

"They have always flown with AirAsia and there was no problem. I am very worried that the plane might have crashed."

Indonesia's President Joko Widodo said his nation was "praying for the safety" of those onboard.

His country, a vast archipelago with poor land transport infrastructure, has seen an explosive growth in low-cost air travel over recent years.

But the air industry has been blighted by poor safety standards in an area that also experiences extreme weather.

AirAsia said the missing jet last underwent maintenance on November 16. The company has never suffered a fatal accident.

It swiftly replaced its distinctive bright red logo with a grey background on its social media pages.

An official from Indonesia's transport ministry said the pilot asked to ascend by 6,000 feet to 38,000 feet to avoid heavy clouds.

"The plane is in good condition but the weather is not so good," Djoko Murjatmodjo told a press conference at Jakarta's airport, addressing reports of severe storms in the area where the jet went missing.

Climbing to dodge large rain clouds is a standard procedure for aircraft in these conditions.

"What happens after that is a question mark," according to Indonesian-based aviation analyst Dudi Sudibyo.

The White House said US President Barack Obama had been briefed on the disappearance and that it was monitoring the situation.

The plane's disappearance comes at the end of a disastrous year for Malaysian aviation.

Malaysia Airlines Flight MH370, carrying 239 people, vanished in March after inexplicably diverting from its Kuala Lumpur-Beijing course. No trace of it has been found.

Another Malaysia Airlines plane went down in July in rebellion-torn eastern Ukraine, killing all 298 aboard. It was believed to have been hit by a surface-to-air missile.

AirAsia has seen spectacular success and aggressive growth under Fernandes' low-cost, low-overheads model.