The perjury trial has opened for a former top White House aide accused of lying to investigators looking into who leaked the identity of a CIA officer.

The defendant, Lewis "Scooter" Libby, is the former chief of staff to Vice President Dick Cheney.

On Tuesday, Libby's lawyer argued the White House tried to blame Libby for the leak in order to protect President Bush's top political strategist, Karl Rove. He said Libby is innocent.

But lead prosecutor Patrick Fitzgerald said that Libby lied repeatedly to investigators and to the grand jury, obstructing an investigation into who revealed the identity of CIA operative Valerie Plame to reporters in July 2003.

Plame's husband, former ambassador Joseph Wilson, had criticized the Bush administration's use of intelligence before the 2003 invasion of Iraq.

Cheney is expected to testify during the trial, which is expected to last about four weeks.

Plame and Wilson have filed a civil lawsuit against Cheney and other U.S. officials. They allege Cheney leaked Plame's classified identity in retaliation for Wilson's public criticism of administration claims that Iraq was seeking nuclear weapons material from Niger.

Wilson went public with his criticism, saying there was no credible evidence for the attempted uranium purchase, in July of 2003. Days later, Plame's name appeared in a syndicated newspaper column.

Some information for this report provided by AP, AFP, Reuters.