WASHINGTON, Feb 9 (Reuters) - Former U.S. Vice President Mike Pence and former national security adviser Robert O'Brien have been subpoenaed by the special counsel leading probes into classified documents found at former President Donald Trump's Mar-a-Lago residence and efforts to overturn the 2020 election result, according to media reports on Thursday.

Pence was issued a subpoena by special counsel Jack Smith, though the nature of the request was not immediately known, ABC News reported, citing sources. The action follows months of negotiations involving federal prosecutors and Pence's lawyers.

O’Brien has been asserting executive privilege in declining to provide some of the information that prosecutors are seeking from him, according to CNN.

Pence's office did not immediately respond to a request for comment. Smith's office declined to comment on both reports from CNN and ABC.

Trump's former acting Department of Homeland Security Secretary Chad Wolf was interviewed by Justice Department lawyers in recent weeks as part of the ongoing special counsel investigation related to 2020 election interference, the report added, citing sources.

U.S. Attorney General Merrick Garland named Smith as special counsel in November to oversee investigations of Trump, shortly after Trump said he would seek the Republican nomination for president again in 2024.

The first probe involves Trump's handling of highly sensitive classified documents he retained at his Florida resort after leaving the White House in January 2021.

The second investigation is looking at efforts to overturn the 2020 presidential election's results, including a plot to submit phony slates of electors to block Congress from certifying Democrat Joe Biden's victory.

Grand juries in Washington have been hearing testimony in recent months for both investigations from many former top Trump administration officials.

Last month, Garland named a separate special counsel, Robert Hur, to probe the improper storage of classified documents at President Biden's home and former office.

In late January, Pence said he was not aware though he takes "full responsibility" after classified documents were found at his Indiana home.

The documents were discovered after a review of his personal records was conducted in the wake of classified material being found at President Joe Biden's home in Delaware.