BEIJING, March 20 (Reuters) - Separatist militants who tried to attack a Chinese domestic flight early this month came from Pakistan and Central Asia, sources said, adding that the apparent bungled assault had international backing.

Chinese officials have said the March 7 incident, in which a plane on its way from the restive, predominately Muslim region of Xinjiang to Beijing abruptly cut short its journey and landed in Lanzhou, a northwest city, involved a foiled assault by passengers, but they have revealed few details.

The Communist Party chief of Xinjiang, Wang Lequan, said this week that the incident was a failed attack by separatists based abroad seeking an independent Xinjiang.

Now one source with direct knowledge of the official Chinese inquiry has told Reuters that the chief suspects -- a man and a woman -- boarded the flight as Pakistani nationals.

"The woman was carrying flammable liquids and evaded security checks by going through the first-class boarding area," said one source, who has spoken to investigators.

"They were carrying Pakistani passports. That does not mean they've concluded they were Pakistani nationals. The passports may have been fake or illegally obtained."

But the source also said the woman had been born in Xinjiang and spent many years in Pakistan, where Islamic militants have detonated numerous suicide bombs.

A second source said the woman was a young Uighur who was trained by a Pakistan-based militant group, while the man was from Central Asia and in his 30s. (Editing by Nick Macfie)

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