US Vice President Mike Pence has hired a personal lawyer to deal with requests from the special counsel investigating possible collusion with Russia during the 2016 election, a top aide said.

"The Vice President has retained Richard Cullen of McGuire Woods to assist him in responding to inquiries by the special counsel," Pence communications director Jared Agen said in a statement.

"The vice president is focused entirely on his duties and promoting the president's agenda and looks forward to a swift conclusion of this matter."

Mr Pence's announcement comes after several weeks of deliberations, featuring several candidates, an administration official said, adding that no taxpayer money will be used.

A final decision was made to hire Mr Cullen earlier this week.

Mr Cullen - who has experience litigating the Iran-Contra investigation, Watergate and the 2000 vote recount in Florida - is a former US attorney for the Eastern District of Virginia.

Mr Pence's move follows a decision by President Donald Trump to hire outside counsel Marc Kasowitz and could signal the investigation is entering a new phase.

Amid reports that special counsel Robert Mueller is also looking into whether Mr Trump tried to obstruct justice, several White House aides are expected to be contacted by his team and are in the process of "lawyering up."

Presiden Trump said on twitter that after months of investigations into possible collusion by his campaign with Russia in the 2016 presidential election campaign, "nobody has been able to show any proof."