US warplanes again struck air defence targets in northern Iraq on Thursday, the first time since major raids were launched near the capital Baghdad last week. US forces' European Command said the strike was in retaliation for anti-aircraft artillery being fired at aircraft patrolling the air exclusion zone imposed by the UK-US coalition which covers much of the north of the country, the US command is quoted by BBC as saying. Last Friday's bombing was the first time targets near the Iraqi capital have been attacked for more than two years, although there have been numerous attacks on outlying regions in response to the threat of Iraqi artillery fire against coalition planes. In the meantime, US President George W. Bush has protested to China about its alleged involvement in bolstering Iraq's air defences. At his first news conference since taking office, Mr. Bush said he was "concerned about the Chinese presence in Iraq" and was "sending the appropriate response" to Beijing. China has already denied allegations that Chinese civilian and military workers have been helping lay fibre-optic cables to improve Iraq's air defence network, in contravention of UN sanctions.