BAGHDAD, Dec. 17 — Prime Minister Tony Blair of Britain pledged his support for the Iraqi government in a surprise visit here on Sunday. Less than a mile from where he spoke, gunmen in police uniforms seized 25 employees of an Iraqi aid organization. Mr. Blair said the preparations to give control of Basra, the southern city where the British military is based, to Iraqi troops were “going well.” But he added that British troops would remain in Iraq “until the job is done” and the Iraqi Army could stand on its own. “We stand ready to support you in every way,” Mr. Blair said at a joint news conference with Prime Minister Nuri Kamal al-Maliki in the Green Zone. He flew to Basra later on Sunday.