US President Donald Trump's eldest son, Donald Trump Jr, and his former campaign manager Paul Manafort are slated to testify before the Senate Judiciary Committee on July 26. 

The news of the open hearing came as US investigators continue their probe into allegations that officials with the Trump campaign colluded with Moscow to influence the race to the White House in favor of the real estate mogul.  

The public hearing is entitled "Oversight of the Foreign Agents Registration Act and Attempts to Influence US Elections: Lessons Learned from Current and Prior Administrations" and follows confirmation from Trump Jr that he met with a Kremlin-linked Russian, who, he was told, possessed incriminating information about his father's presidential opponent Hillary Clinton in the run-up to the November 8 elections.

The controversial meeting was also attended by Manafort.

Adding to the political drama in Washington next week, Trump's son-in-law Jared Kushner will testify before the Senate Intelligence Committee on Monday, albeit in a closed-door session, Reuters reported citing Kushner's attorney.

Kushner, who is married to Trump's daughter Ivanka Trump, also attended the June 2016 meeting with the Russian lawyer, Natalia Veselnitskaya.

Trump Jr sent shockwaves through Washington when he confirmed the meeting by releasing a series of emails in which the now-president's son was told the campaign could get "very high level and sensitive information" that was "part of Russia and its government's support for Mr Trump."

It put Trump's son and son-in-law at the center of a burgeoning scandal involving multiple US investigations into whether Trump associates colluded with Moscow in its efforts to tilt the 2016 election in the Republican's favor.

Judiciary Committee Chairman Chuck Grassley and top Democrat Dianne Feinstein warned in letters to those testifying next week that they need to "preserve all relevant documents in your possession... related to Russian interference in the 2016 election, including documents related to your or the Trump campaign's contacts with Russian government officials, associates or representatives."

The senators said they would issue subpoenas if the witnesses did not produce the required documents.

(With inputs from Reuters and AFP)