The South had scheduled to ship 4,000 tons of cement to North Korea on Tuesday, but decided to delay it "under the current circumstances," Yonhap quoted an unidentified Unification Ministry official as saying.
North Korea alarmed the world earlier Monday by declaring that it successfully conducted its first nuclear bomb test.
South Korean President Roh Moo-hyun said Monday that North Korea's claimed nuclear weapons test was a serious threat to regional stability.
North Korea's reported test "is a severe situation that threatens stability on the Korean Peninsula and in northeast Asia," Roh told journalists after a summit with new Japanese Prime Minister Shinzo Abe. "South Korea will react sternly and calmly" in close cooperation with the international community, he said.
"Our military, based on the (South) Korea-U.S. alliance, has the capability to cope with any provocation by the North," Roh said.