ISLAMABAD, Pakistan — The uneasy truce between the Pakistani government and Taliban militants in the Swat Valley appeared increasingly fragile on Monday as government forces attacked militants in a neighboring district for a second day, causing the main negotiator for the Taliban to break off talks. Maulana Sufi Muhammad, the pro-Taliban cleric who has been negotiating peace talks between the government and the Taliban in Swat, halted talks on Monday to protest the military operation in the Lower Dir district west of Swat, his spokesman said.