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PESHAWAR, Pakistan, June 16 (Reuters) - A bomb exploded in a Shi'ite Muslim mosque in Pakistan's northwestern city of Dera Ismail Khan on Monday, killing four people and wounding two, police said.

The bomb went off at dusk when Shi'ites were gathered in the mosque for prayers.

"It appears to be a time-device," police official Abdul Ghaffar told Reuters.

He said the bomb was planted in a toilet adjacent to the mosque.

No one claimed responsibility for the attack. Ghaffar said he suspected militants fighting security forces in the neighbouring tribal region of South Waziristan were behind the attack.

Pakistan has seen a wave of militant violence since July last year and hundreds of people have been killed in the attacks.

But, the violence has dipped recently after the new government opened talks with al Qaeda-linked militants based near the Afghan border.

Members of Pakistan's Sunni Muslim majority and minority Shi'ites largely co-exist peacefully. However, militant groups from both sides have killed thousands of people in tit-for-tat attacks over nearly three decades. (Reporting by Alamgir Bitani; Writing by Kamran Haider; Editing by Zeeshan Haider/Keith Weir)

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