Democratic vice presidential candidate Tim Kaine on Sunday defended Hillary Clinton against criticism over her handling of classified information as secretary of state, saying she was unaware of the sensitivity of some information she exchanged over email because it had been "improperly labeled."

In July, the Federal Bureau of Investigation rebuked Clinton, the Democratic presidential nominee, for her use of a private email server during her tenure as secretary of state from 2009 to 2013, saying she was "extremely careless" in her handling of classified information.

According to records released by the FBI on Friday, Clinton told the bureau's investigators in a July interview that she could not recall getting any briefings on how to handle classified information or comply with laws governing the preservation of federal records.

Kaine, a U.S. senator from Virginia who sits on the Senate Foreign Relations and Armed Services committees, told ABC's program "This Week," that the volume of information received by government officials often makes it difficult to know what information has been declared classified.

"There were emails that contained classified information that had been improperly marked. So when she received the email, the material that was classified - which is supposed to be flagged and identified as classified - in many instances was improperly labeled," Kaine said.

"We look at so much material, unless it is specifically pulled out and identified, it is difficult to know sometimes whether a statement or a paragraph is classified or not," he added.

Kaine likened Republican Donald Trump's recent call for Russia to "find the 30,000 emails that are missing" from Clinton's server to the 1972 burglary at the Democratic Party headquarters at the Watergate office in Washington that upended Richard Nixon's presidency.

"He has openly encouraged Russia to engage in cyber hacking to try to find more e-mails or materials, and we know that this cyber attack on the DNC was likely done by Russia," Kaine told the Sunday television program. "This is serious business."

Trump was referring to some 30,000 emails deemed personal by Clinton's lawyers that she has repeatedly said she chose "not to keep" while returning thousands of other work emails to the government in 2014. This has been widely interpreted as meaning that Clinton deleted the personal emails, but her staff have declined to confirm if this is the case.

Clinton, who is facing Trump for the White House in the Nov. 8 election, has been dogged for more than a year by the fallout from her decision to use an unauthorized private email account run from the basement of her Chappaqua, New York, home.

She repeatedly said she did not use it to send or receive classified information. The government forbids handling such information outside secure channels.

Contradicting Clinton's comment that she never exchanged classified information over her private email server, the FBI said that at least 81 email threads contained information that was classified at the time, although the agency said the final number may be more than 2,000. While the FBI has scolded Clinton over the handling of classified information, the agency recommended that no criminal charges be filed against her.

Clinton has said that in hindsight she regretted using a private email system while secretary of state. Opinion polls show that voter concerns about Clinton's honesty and trustworthiness are among her biggest vulnerabilities.

A series of surveys show the race has tightened over the past few weeks. In a Reuters/Ipsos poll taken during the week of Aug. 26 to Sept. 1, Trump had the support of 40 percent of likely voters while Clinton had the backing of 39 percent. Clinton's support has dropped steadily in the weekly tracking poll since Aug. 25, eliminating what had been a eight-point lead for her.

(Editing by Caren Bohan and Alan Crosby)