Congressional Democrats have taken legal action to gain access to all of US Special Counsel Robert Mueller's evidence from his inquiry into Russian interference in the 2016 election.

It comes as the number of Americans who approve of US President Donald Trump dropped by 3 percentage points to the lowest level of the year, according to a Reuters/Ipsos online opinion poll.

Mr Mueller did not establish the Trump campaign co-ordinated with Russians but did find "multiple acts by the President that were capable of exerting undue influence over law enforcement investigations."

While Mr Mueller ultimately decided not to charge Mr Trump with a crime such as obstruction of justice, he also said the investigation did not exonerate the president, either.

US House of Representatives Judiciary Committee Chairman Jerrold Nadler, a Democrat, issued a subpoena to the Justice Department to hand over the full Mueller report and other relevant evidence by 1 May.

"My committee needs and is entitled to the full version of  the report and the underlying evidence consistent with past practice. The redactions appear to be significant," Mr Nadler said in a statement.

The Justice Department called the request "premature and unnecessary," but spokeswoman Kerri Kupec said in a statement the department would work with Congress "to accommodate its legitimate requests consistent with the law and long-recognised executive branch interests."

The report provided extensive details on Mr Trump's efforts to thwart Mr Mueller's investigation, giving Democrats plenty of political ammunition against the Republican president but leaving them with no consensus on how to use it.

The document has blacked out sections to hide details about secret grand jury information, US intelligence gathering and active criminal cases as well as potentially damaging information about peripheral players who were not charged.

Six top congressional Democrats, led by House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer, rejected US Attorney General William Barr's offer to give them access to a less-redacted version of the report.

In a letter to Mr Barr, they repeated their request for the full report but said they were open to "a reasonable accommodation."

Democratic leaders have played down talk of impeachment of Mr Trump just 18 months before the 2020 presidential election, even as some prominent members of the party's progressive wing, notably US Representative Alexandria Ocasio-Cortez, promised to push the idea.

US Senator Elizabeth Warren became the first major contender for the Democratic 2020 presidential nomination to call for the start of impeachment proceedings, saying on Twitter that "the severity of this misconduct" demanded it.

Mr Trump, who has repeatedly called the Mueller probe a political witch hunt, lashed out again yesterday.

"Statements are made about me by certain people in the Crazy Mueller Report...which are fabricated & totally untrue," Mr Trump wrote on Twitter.

He seemed to be referring to former White House counsel Don McGahn, who was cited in the report as having annoyed Mr Trump by taking notes of his conversations with the president.

"Watch out for people that take so-called 'notes,' when the notes never existed until needed," Mr Trump wrote.

"It was not necessary for me to respond to statements made in the 'Report' about me, some of which are total bullsh*t & only given to make the other person look good (or me to look bad)."

Phone conversations between the president and Mr McGahn in June 2017 were a central part of Mr Mueller's depiction of Mr Trump as trying to derail the Russia inquiry.

The report said Mr Trump told Mr McGahn to instruct the Justice Department to fire Mr Mueller. Mr McGahn did not carry out the order.

According to the Reuters/Ipsos poll of 1,005 adults conducted Thursday afternoon to yesterday morning, 37% of people approve of Mr Trump’s performance in office - down from 40% in a similar poll conducted on 15 April, which matches the lowest level of the year.