There were conflicting accounts of how the attack happened, and police said they were still investigating.
But one official said the bomber drove onto the field as people gathered to watch a match.
At least one home near the sports field collapsed, with people said to be trapped under rubble.
The attack took place in Lakki Marwat, close to the tribal areas of North and South Waziristan.
Earlier, one police official told Reuters that a suicide bomber had approached the game on foot and blown himself up.
"There´s a lot of damage. Roofs of nearby houses collapsed. There could be high casualties," the official, Habibullah Khan, said.
Mr Khan was quoted by the Associated Press as saying the attack may have been in retaliation for attempts by locals to expel militants by setting up their own militia.
North and South Waziristan form a lethal militant belt from where insurgents have launched attacks across north-west Pakistan as well as into parts of eastern Afghanistan.
Since October, the Pakistani army has been carrying out a ground offensive against militants in South Waziristan.
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