RICHMOND, Virginia (Reuters) - Mitt Romney accused Vice President Joe Biden on Friday of contradicting the testimony of U.S. State Department officials on Libya, in an escalation of the Republican presidential challenger's attacks over the deaths of four Americans there.

Hoping to puncture President Barack Obama's credibility on foreign policy ahead of the November 6 election, Romney jumped on comments that Biden made on Thursday night during a debate with Romney's vice presidential running mate, Paul Ryan.

When asked about whether the U.S. diplomatic compound in Benghazi, Libya had asked for more security ahead of the attack, Biden said: "Well, we weren't told they wanted more security again. We did not know they wanted more security again."

Two State Department officials gave sworn testimony on Wednesday at a congressional hearing in Washington saying they had repeatedly requested beefed-up security for the compound before U.S. Ambassador Christopher Stevens and three other Americans were killed in the assault at the site on September 11.

"The vice president directly contradicted the sworn testimony of State Department officials," Romney told a campaign rally in Richmond. "He's doubling down on denial."

"When the vice president of the United States directly contradicts the testimony, the sworn testimony of State Department officials, American citizens have a right to find out what's going on," he said.

(Editing by Alistair Bell and Will Dunham)