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* Taliban say blast on base caused by infiltrator
* Bomb in south kills three nephews of Karzai ally
By Sayed Salahuddin
KABUL, April 19 (Reuters) - An explosion at an Afghan army base near Kabul's airport on Monday killed one NATO service member and one Afghan soldier, officials said, in an attack claimed by the Taliban.
A separate bomb attack killed three young nephews of a high-profile ally of President Hamid Karzai in the southern city of Kandahar, where violence has surged ahead of a planned major offensive by U.S.-led forces.
Afghan and NATO officials declined to give details of the cause of the blast at the base near Kabul airport, saying it was still under investigation.
Taliban spokesman Zabihullah Mujahid said the attack was carried out by a Taliban infiltrator, who struck while foreign advisers were training Afghan troops.
The airport is a combined civil-military one which is also used by foreign troops and in the past has come under rocket attacks as well as suicide strikes at its gates.
A Defence Ministry spokesman Zaher Murad said one Afghan soldier was killed and three wounded. The NATO-led International Security Assistance Force said in a statement that one of its members was also killed and several wounded.
NATO efforts to train Afghan forces have been plagued in recent months by "rogue" Afghan soldiers and police turning weapons on their trainers, and infiltrators gaining access to compounds meant to be secure.
In November, an Afghan police trainee killed five British soldiers at a checkpoint. In December, a Jordanian infiltrator at a U.S. base killed seven employees or contractors working for the CIA and a Jordanian intelligence officer.
In the other big attack on Monday, a bomb strapped to the back of a donkey killed three young nephews of a tribal chief allied to Karzai in Kandahar, the president's home town.
Three pedestrians and two police were also wounded in the blast, which struck a police post guarding the home of the tribal chief, Fazluddin, said Zalmai Ayoubi, spokesman for the regional governor.
The city, birthplace of the resurgent Taliban, has witnessed a dramatic upsurge in violence in recent weeks, ahead of a planned major offensive by U.S.-led NATO troops.
U.S. officials say their operation in Kandahar in coming months will be the centrepiece of a military campaign this year to turn the tide in the eight-year-old war with an extra 30,000 troops sent by President Barack Obama. (Additional reporting by Ismail Sameem in KANDAHAR; writing by Sayed Salahuddin and Peter Graff; Editing by Jeremy Laurence) (For more Reuters coverage of Afghanistan and Pakistan, see: here) (firstname.lastname@example.org; Kabul newsroom: +93 799 335 285)) (If you have a query or comment about this story, send an e-mail to email@example.com)