BAGHDAD — Kurdish security forces took over two major oil fields outside the disputed northern city of Kirkuk before dawn Friday, Iraq’s Oil Ministry said, the latest move in a deepening a dispute with the government of Prime Minister Nouri al-Maliki. Oil Ministry spokesman Assem Jihad denounced the takeover of the Bai Hassan and Kirkuk oil fields as “a violation to the constitution” and warned that it poses “a threat to national unity.” He said Kurdish troops moved in and expelled local workers from the two sites. The seizure of the fields could accelerate the unraveling of already worsening relations between the Kurdish autonomy zone in the north and Iraq’s central government. The spat is one of the ripple effects of the Sunni militant offensive that overran much of northern and western Iraq last month, plunging the country into its worst crisis since the last U.S. troops left in 2011.