TANK, Pakistan, June 15 (Reuters) - A passenger bus fell into a ravine near Pakistan's South Waziristan tribal region on Friday, killing at least 13 people and wounding more than three dozen, an official said.

The bus was travelling from Waziristan to Dera Ismail Khan, a town about 300 km (185) miles northwest of the capital, Islamabad, when it veered off the mountainous road at a sharp bend.

"The accident occured due to over-speeding and at least 13 people have been killed," Waziristan's senior administrator, Hussain Zada, told Reuters.

Hospital officials at Waziristan's Jandola district said 15 people were killed. ((Writing by Augustine Anthony, editing by John Chalmers; Reuters Messaging: augustine.anthony.reuters.com@reuters.net; +92-51-281 0017))

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