Four Palestinian children were killed and one was critically wounded on a Gaza beach on Wednesday by a shell fired by an Israeli naval gunboat, a Palestinian health official said.Asked about the incident, an Israeli military spokesman in Tel Aviv said he was checking the report."The kids were playing football on the beach. They were all ... under the age of 15," said Ahmed Abu Hassera, 22."When the first shell hit the land, they ran away but another shell hit them all," said Abu Hassera, whose shirt was stained with blood."It looked as if the shells were chasing them," he told Reuters.Ashraf al-Qidra, spokesman for the Gaza Health Ministry, said four of the youngsters were killed and a fifth youth was in critical condition. "This is a cowardly crime," he said.The Gaza beach area has been shelled frequently since Israel began its offensive in the enclave on July 8 following a surge in Palestinian cross-border rocket strikes.Gaza health officials said 207 Palestinians, most of them civilians, had been killed in air and naval barrages, in the worst flareup of Israeli-Palestinian violence in two years.One Israeli has been killed by shelling from Gaza that has made a race to shelter a daily routine for hundreds of thousands in Israel.