Pence's announcement on Thursday followed several weeks of deliberations, an administration official said, adding that no taxpayer money would be used.

"The vice president has retained Richard Cullen of McGuire Woods to assist him in responding to inquiries by the special counsel," Pence's 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."

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.

Investigators working under special counsel Robert Mueller since he was appointed by the US Justice Department in May to take over the probe into possible Russian meddling in the 2016 election are planning to speak with key intelligence officials on the matter.

Mueller is also looking into the finances and business dealings of presidential adviser Jared Kushner, The Washington Post reported. 

Kushner - Trump's son-in-law - is one of several people close to the president who are "people of interest" in the investigation.

Others include former Trump campaign chairman Paul Manafort and adviser Carter Page.

Kushner is of interest because of meetings he had with Russian ambassador Sergey Kislyak and a Russian banker after the election and before Trump took office. 

Kushner also allegedly suggested setting up a secret and secure communications channel between the US president's transition team and Moscow.

Questions over what Pence knew, when

On Wednesday, The Washington Post reported that Trump may in fact be investigated by Mueller for possible obstruction of justice. Trump's personal lawyer, Marc Kasowitz, issued a statement saying the FBI was behind the story and called the leak "outrageous, inexcusable and illegal."

Senators on Tuesday questioned both Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein.

Pence has stood by his March statement that he first learned of the now-fired national security adviser Michael Flynn's ties to Turkey from media reports. Flynn told the Trump transition team more than two weeks before the inauguration that he was under federal investigation for his work as a lobbyist advocating for Turkish government interests.

The question of what Pence knew and when arose after reports surfaced that Flynn had discussed sanctions with Russia's ambassador to Washington, despite Pence's public claim to the contrary.

The Comey question

Pence also claimed that Trump fired former FBI Director James Comey in May because of a memo written by the deputy attorney general. Trump subsequently said he planned to fire Comey regardless.

Pence said in early June that he did not think an independent prosecutor was necessary to investigate potential collusion between the Trump campaign and the Russian government. He defended Comey's ouster against accusations that it was designed to halt the Russia investigation.

Sessions under pressure?

An American lobbyist for Russian interests - Richard Burt, an ex-ambassador to Germany during the Reagan administration who helped prepare a foreign policy speech for Trump - told The Guardian newspaper on Thursday he had attended two dinners hosted by Attorney General Jeff Sessions during the 2016 campaign.

Sessions testified under oath this week that he "did not believe" he had had any contacts with lobbyists working for Russian interests during Trump's campaign.

jbh/cmk (AFP, Reuters, dpa)