QUETTA, Pakistan (Reuters) - Pakistan's Lashkar-e-Jhangvi, which intelligence officials say has become a major security threat, claimed responsibility for a sectarian attack on Shi'ites which killed 47 people in the city of Quetta on Saturday.

Officials said most of the dead were from Pakistan's Shi'ite minority, which has come under siege from Sunni extremist groups who seem to carry out sectarian attacks at will.

"The explosion was caused by an improvised explosive device fitted to a motorcycle," said Wazir Khan Nasir, deputy inspector general of police in Quetta. He said 130 people were wounded.

"This is a continuation of terrorism against Shi'ites."

More than 400 Shi'ite were killed in Pakistan last year, many by hitmen or bombs, and the perpetrators are almost never caught.

A spokesman for the LeJ claimed responsibility for Saturday's bloodshed near the main bazaar, school and computer centre. Burned school bags and books were strewn around.

"I saw many bodies of women and children," said an eyewitness at a hospital. "At least a dozen people were burned to death by the blast."

Last month, LeJ said it carried out a bombing in Quetta that killed nearly 100 people, one of Pakistan's worst sectarian attacks. Thousands of Shi'ites protested in several cities after that attack.

Most Western intelligence agencies have regarded the Pakistani Taliban and al Qaeda as the gravest threat to nuclear-armed Pakistan, a strategic U.S. ally.

But Pakistani law enforcement officials say LeJ has become a formidable force, staging attacks in several parts of the country in a bid to spark sectarian warfare, create chaos and install a Sunni theocracy.

The growing sectarian violence has hurt the credibility of the government, which has already faced criticism ahead of elections due in May for its inability to tackle poverty, corruption and economic stagnation.

(Reporting by Jibran Ahmed and Gul; Writing by Michael Georgy)