Uzbek militants have a reputation for ferocity (File photo)
In South Waziristan, the Pakistani army is bracing itself for confrontation with what it says are "a large group of Uzbek extremists". So who are they and what are they doing in Pakistan? Sirojiddin Tolibov of the BBC's Uzbek service has this assessment.
Most of the Uzbek militants in South Waziristan belong to the al Qaeda-linked group, the Islamic Movement of Uzbekistan (IMU).
Renowned for their fanaticism, Uzbek militants initially fled their home country in the early 1990s after a government crackdown on people who advocated the introduction of Sharia law in secular Uzbekistan.
Nobody knows exactly how many there are in Pakistan - estimates vary wildly from 500 to 5,000. Not all Uzbeks there are active militants - some are merely supporters of the Taliban while others are little more than "hired guns".
When founded in August 1998 in the north of Afghanistan, the IMU's main aim was to overthrow the government of President Islam Karimov and establish an Islamic state in Uzbekistan.
In 1999 the IMU set up several military camps in northern Afghanistan from where it launched incursions into southern Kyrgyzstan and Uzbekistan in 1999 and 2000.
When the US-led alliance invaded Afghanistan in 2001, the IMU announced its loyalty to the Taliban.
The IMU is believed to have suffered heavy losses while fighting alongside the Taliban against US-led forces in 2001. But it successfully re-organised itself in the tribal areas of North and South Waziristan over the following years.
A former IMU member told the BBC Uzbek service that there were more than 100 families in the tribal areas by the end of 2001.
Most of them were children and widowed women. Among them there were several hundred IMU members. The authorities in Islamabad last month claimed that the army killed the group's leader, 42-year-old Tahir Yuldashev, but this has never been confirmed.
The overall picture is further confused because some Pakistani officials erroneously assume that Islamic fighters from other countries - such as Chechnya - are from Uzbekistan.
Although Central Asian governments continue to regard the organisation as the main threat to the region's stability, its capability as an operational force outside Pakistan and Afghanistan is questioned by analysts.
The Pakistani army faces a formidable opponent
The Uzbek contingent is thought to have mixed with the local population to some extent, becoming a permanent fixture of the militant scene in Pakistan's north-western border areas.
Experts say that the Uzbeks were first targeted by Pakistani security forces in March 2007 when militants started attacking and killing Pakistani soldiers and paramilitary troops.
Since then Tahir Yuldashev's men have been accused of numerous suicide attacks and of killing hundreds of tribal elders in Pakistan over the years.
His militants carried out attacks on Pakistani forces in the tribal belt and are believed to have taken part in the Red Mosque siege in 2007 in which dozens of people were killed.
The military say that Uzbek militants are a formidable enemy.
Pakistani security forces believe that some of the major bomb attacks in Islamabad are the work of Central Asian militants.