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Kim Kwan-Jin told lawmakers the drills began after the ice started to thaw and warned of the possibility of "various types of surprise local provocations" targeting the South.
"There are also frequent visits by (the North's) senior commanders to military posts and activities to keep (troops) ready for operations," Kim said.
He added the North's military continued its usual operations despite severe food and fuel shortages plaguing the impoverished country.
Cross-border military tension has been acute since the North's November shelling of an island near the flashpoint border off the west coast that killed four South Koreans including two civilians.