China's Foreign Ministry on Wednesday fired back at US complaints over the harassment of a US Navy mapping ship by Chinese boats in international waters over the weekend. In a statement posted on the Foreign Ministry's Web site, spokesman Ma Zhaoxu called "unacceptable" US claims that the USNS Impeccable was operating legally within China's exclusive economic zone when the encounter occurred Monday. US Defense Department officials said the ship is designed and equipped for sub-hunting work and was part of a calculated US surveillance operation in the disputed South China Sea. The US claims were "gravely in contravention of the facts and unacceptable to China," Ma said in the statement, which mainly reiterated comments he made at a news conference on Tuesday. "China has lodged a solemn representation to the United States as the USNS Impeccable conducted activities in China's exclusive economic zone in the South China Sea without China's permission," he said. China has demanded the US take "effective measures to prevent similar acts from happening," he added.