ISLAMABAD, July 17 (Reuters) - A blast was heard in the Pakistani capital Islamabad on Tuesday, Reuters witnesses reported, and a television station said at least 4 people were killed.

It was not immediately clear what caused the blast.

Pakistan has seen a surge in violence since government forces stormed Islamabad's Lal Masjid, or Red Mosque, compound earlier this month ending a week-long siege and killing supporters of a hardline cleric.

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