Q+A - What is the Pakistani Taliban trying to achieve?

(Reuters) - A suicide bomber killed at least 20 people in the Pakistani city of Rawalpindi on Monday as security forces pressed ahead with their offensive to wrest control of the Taliban strongholds in their South Waziristan bastion on the Afghan border.

More than 150 people were killed in a wave of attacks by the militants across the country before the Waziristan offensive was started on the Oct. 17.

Here are some questions and answers about the violence.

WHAT ARE THE TALIBAN TRYING TO DO?

The Taliban want to impose their hardline version of Islamic rule and expel U.S. forces from the region. They are also exacting revenge for the deaths of their "martyrs", in particular their leader, Baitullah Mehsud, who was killed in a missile attack by a pilotless U.S. drone on Aug. 5.

The government says the militants, who have close links with Afghan Taliban, al Qaeda and anti-Indian militant groups, are hoping the stepped-up urban attacks will weaken the resolve of authorities to tackle the issue and also distract them from the offensive.

Some analysts say the wave of violence is brash new Pakistani Taliban leader Hakimullah Mehsud trying to make his mark. People who have met him say Hakimullah is more reckless and ruthless than his predecessor.

In comments released on a video this month, Hakimullah, 31, said his fighters would stop their attacks and would fight India instead if the government "stopped following U.S. orders".