* China says Uighurs militants hiding in Pakistan
* Pakistan says doing everything to weed out insurgency
* Separatist leader threatens attacks against China
By Saud Mehsud and Maria Golovnina
DERA ISMAIL KHAN/ISLAMABAD, March 14 (Reuters) - Entrenched in secret mountain bases on Pakistan's border with Afghanistan, Uighur fighters are gearing up for retribution against China to avenge the deaths of comrades in Beijing's crackdown on a separatist movement, their leader told Reuters.
China, Pakistan's only major ally in the region, has long urged Islamabad to weed out what it says are militants from its western region of Xinjiang, who are holed up in a lawless tribal belt, home to a lethal mix of militant groups, including the Taliban and al Qaeda.
A mass stabbing at a train station in the Chinese city of Kunming two weeks ago, in which at least 29 people were killed, has put a new spotlight on the largely Muslim Uighur ethnic minority from Xinjiang, where Beijing says armed groups seek to establish an independent state called East Turkestan.
Beijing has called the Kunming bloodshed a "terrorist attack" carried out by militants, and says separatists operate training camps across the rugged border which abuts Pakistan and Afghanistan.
In a rare but brief interview, Abdullah Mansour, leader of the rebel Turkestan Islamic Party, said it was his holy duty to fight the Chinese.
"The fight against China is our Islamic responsibility and we have to fulfil it," he said from an undisclosed location.
"China is not only our enemy, but it is the enemy of all Muslims ... We have plans for many attacks in China," he said, speaking in the Uighur language through an interpreter.
"We have a message to China that East Turkestan people and other Muslims have woken up. They cannot suppress us and Islam any more. Muslims will take revenge."
Mansour spoke on a crackly line using a mobile phone with an Afghan SIM card in a brief statement which gave Reuters no chance to ask about the Kunming attack.
The separatists hide mainly in the troubled North Waziristan region, where they are treated by their Pakistani Taliban hosts as guests of honour, militant and Pakistani intelligence sources say.
The Turkestan Islamic Party, which China equates with the East Turkestan Islamic Movement (ETIM), keeps a low profile in Pakistan. Unlike the Taliban, it almost never posts videos promoting its activities or ideology. Its exact size is unknown and some experts dispute its ability to orchestrate attacks in China, or that is exists at all as a cohesive group.
Getting hold of leaders such as Mansour is almost impossible and interviews are usually very brief and conducted from undisclosed locations through a Pashto-speaking translator.
Pakistani intelligence sources say they number about 400 fighters, and are clustered around the remote Mir Ali area, sharing bases with other foreign insurgents, particularly Uzbeks, who speak a similar language.
In Afghanistan, two security reports sent to expatriates working there this year warned of attacks on a Chinese hotel, Chinese companies and other targets in Kabul. There have been no attacks so far.
According to Afghan Taliban sources, there are about 250 Uighur militants in Afghanistan's Nuristan and Kunar provinces.
"They live here with us but are always concerned about their people and mission in China. They are nice people, good Muslims and the best fighters," a senior Taliban commander said.
He added that Uighur militants were not fond of guns, and resorted mostly to knives and daggers.
China has stepped up security in Xinjiang after a vehicle ploughed into tourists on the edge of Beijing's Tiananmen Square in October, killing the three people in the car and two bystanders. China labelled it a suicide attack by militants from the region.
Mansour released a Uighur-language video weeks after the Tiananmen incident, calling it a "jihadi operation" by its holy warriors.
For Pakistan, China is a valued friend in a region it views as potentially hostile. It is keen to demonstrate a commitment to weeding out what Beijing calls separatists, but its security forces are already stretched fighting Pakistani Taliban militants.
Rehman Malik, Pakistan's former interior minister, said that about 20 Uighur militants were captured and handed over to China on his watch in 2008-2013.
"Pakistan and China are great friends. There are no secrets between us. When I took over as interior minister, I took on this subject in close association with my partners in China," he said. "The present government is also aware of the whole thing."
Many Uighurs in the energy-rich Xinjiang region which borders ex-Soviet Central Asia, Afghanistan, Pakistan and India, accuse Han Chinese of stifling their culture and religion. More than 100 people there have been killed in unrest in the past year, according to Chinese state media reports.
But the Chinese government has provided little evidence that the Kunming killings or any other incidents that Beijing has labelled terrorist attacks have been linked to outside forces.
Some experts have suggested that the low-tech nature of the weapons the assailants used in Kunming and the location of the attack point to a lack of external backing and weakly organised revenge killings as opposed to coordinated international terrorism.
The Kunming attack has put China on edge and prompted concerns over rising discrimination against Uighurs across the country.
Exiled Uighur groups have repeatedly called for transparent investigations into such incidents and say they should not be used as excuses for further repressive policies on Uighur communities.
Hundreds of Uighurs migrated to the lawless areas of Pakistan about five years ago after they were squeezed out of their homeland by a Chinese crackdown, Pakistani security sources say. Their numbers are believed to be much smaller now.
"The Chinese militants in the tribal areas are mostly clerics and fighters. They have their families here and are mostly focused on Afghanistan," said one Pakistani Taliban commander.
Saifullah Mahsud, head of the Pakistani think tank FATA Research Center, which has extensive sources in Pakistan's tribal areas, agreed their power and capacity to carry out major attacks are exaggerated by China.
"It's survival, basically. They can't go back," he said. "This is the only place where they are welcome."
But attempts by Taliban insurgents to carve out new hideouts in northern areas of Pakistan near China's border have helped create a new corridor for Uighurs leading into their homeland.
"In the last couple of years, Taliban militants have got nearer and nearer to the Chinese border," said Mahsud. "There has been a lot of movement there. Perhaps that gives them the logistical support that they require to cross over into China." (Additional reporting by Jibran Ahmed in PESHAWAR, Jessica Donati in KABUL and Michael Martina in BEIJING; Writing by Maria Golovnina in ISLAMABAD; Editing by John Chalmers, Ben Blanchard and Nick Macfie)