The majority of seats on a new committee investigating the 2012 Benghazi attacks will be held by Republicans, a congressional aide said on Tuesday, despite Democrats' insistence that the panel must be evenly divided in order to conduct a nonpartisan investigation.

A senior House of Representatives leadership aide told Reuters that the new House Select Committee will include seven Republicans and five Democrats.

Democrats had called for an evenly divided panel. But Republicans control a majority - 233 - of the 435 seats in the House, and thus would typically have more members on a House panel.

Republicans have led the charge to investigate the administration's handling of the assaults on U.S. diplomatic facilities in Benghazi, Libya, which killed the U.S. ambassador and three other Americans on September 11, 2012.

Eight different congressional committees have investigated the incident, holding more than a dozen hearings and 50 briefings, and examining 25,000 pages of documents.

The intensity of the probe has prompted Democrats to accuse the Republicans of political motivations, with an eye toward tarnishing President Barack Obama and Hillary Clinton, who was secretary of state when the attacks took place and is seen as a likely 2016 Democratic presidential candidate.

Republicans counter that the administration deliberately misled the American public about the nature of the attack as Obama ran for reelection in November 2012.

The release last week of an email by a top Obama foreign policy aide that seemed to suggest a White House effort to protect the president breathed new life into the controversy.

On Friday the House Oversight Committee announced it would issue a rare subpoena to a cabinet member, current Secretary of State John Kerry, to testify on Benghazi. House Speaker John Boehner announced he would seek a vote on forming the select committee.

The Republican-controlled House is expected to easily approve the committee when it comes up for a vote this week.

(Reporting by Patricia Zengerle; Editing by Prudence Crowther)