* Pakistani Taliban say ready to send fighters to help IS

* Regional stability under threat as U.S.-led troops leave

* Message comes after beheading of British aid worker

By Saud Mehsud and Maria Golovnina

DERA ISMAIL KHAN/ISLAMABAD, Oct 4 (Reuters) - The Pakistani Taliban declared allegiance to Islamic State on Saturday and ordered militants across the region to help the Middle Eastern jihadist group in its campaign to set up a global Islamic caliphate.

Islamic State, which controls swathes of land in Syria and Iraq, has been making inroads into South Asia, which has traditionally been dominated by local Taliban insurgencies against both the Pakistan and Afghanistan governments.

The announcement comes after a September move by al Qaeda chief, Ayman al-Zawahri, to name former Taliban commander Asim Umar as the "emir" of a new South Asia branch of the network that masterminded the 2001 attacks on the United States.

Although there is little evidence of a firm alliance yet between IS and al Qaeda-linked Taliban commanders, IS activists have been spotted recently in the Pakistani city of Peshawar distributing pamphlets praising the group.

IS flags have also been seen at street rallies in Indian-administered Kashmir. The trend has been of growing concern to global powers struggling to keep up with the fast-changing nature of the international Islamist insurgency.

In a message marking the Muslim holy festival of Eid al-Adha, the Pakistani Taliban said they fully supported IS goals.

"Oh our brothers, we are proud of you in your victories. We are with you in your happiness and your sorrow," Taliban spokesman Shahidullah Shahid said in a statement sent to Reuters by email from an unknown location.

"In these troubled days, we call for your patience and stability, especially now that all your enemies are united against you. Please put all your rivalries behind you ...

"All Muslims in the world have great expectations of you ... We are with you, we will provide you with Mujahideen (fighters) and with every possible support."

The statement, released in Urdu, Pashto and Arabic, was sent after Islamic State militants beheaded British aid worker Alan Henning in a video posted on Friday, triggering condemnation by the British and U.S. governments.

It also came despite recent speculation that the Taliban leadership, whose goal is to topple the government and set up a Sharia state, is actually wary of IS, which is driven by different ambitions that have little to do with South Asia.

The Pakistani Taliban, funded by local as well as foreign charity donations from wealthy supporters in the Gulf and elsewhere, operate separately from the Afghan insurgents of the same name, but are loosely aligned with them.

There are concerns about further turmoil in the region as most U.S.-led foreign troops withdraw from Afghanistan this year, with groups like the Haqqani network likely to exploit the security vacuum to strengthen their hold on Afghan regions.

The Haqqani network, despite being based in Pakistan, is narrowly focused on its insurgency in Afghanistan and has not commented on IS-related developments.

The Pakistani Taliban have been beset by bitter internal rivalries over the past year, with the influential Mehsud tribal faction of the group refusing to accept the authority of Mullah Fazlullah, who came to power in late 2013.

IS, in an effort to extend its global reach, could exploit these rivalries to its advantage, wading into a region ripe with fierce anti-Western ideology and full of young unemployed men ready to take up guns and fight for Islam. (Editing by Catherine Evans)

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