Baghdad: US Defence Secretary Robert Gates arrived in Baghdad on Monday ahead of a change in Washington's top military leadership in Iraq.

General David Petraeus will transfer responsibility for US forces in Iraq to Lieutenant-General Ray Odierno on Tuesday. Gates plans to attend the ceremony.

Gates said he expects the US combat role to narrow even further and that US troops will increasingly play a backup role in Iraq.

"The areas in which we are seriously engaged [in fighting] will, I think, continue to narrow," Gates said.

Iraq currently has primary responsibility for security in 11 of its 18 provinces.

Gates said he is focusing heavily on expanding the use and effectiveness of intelligence and surveillance programs in Iraq.

"We have a lot more plans under way" for expanding that program, Gates said.