"The South is reconsidering plans to ship 4,000 tons of cement of emergency relief to the North for floods it suffered in mid-July, a Unification Ministry official said on customary condition of anonymity.

"South Korea won't be patient for everything, make concessions on everything and accept all demands from North Korea as it did in the past," Roh said.

Impoverished and isolated North Korea has relied on foreign aid to feed its 23 million people since its state-run farming system collapsed in the 1990s following decades of mismanagement and the loss of Soviet subsidies.

South Korea's Defense Ministry said the alert level of the military had been raised in response to the claimed nuclear test, but that it noticed no unusual activity among North Korea's troops."