WASHINGTON, United States- US President Joe Biden on Thursday pardoned thousands of Americans convicted of marijuana possession in a major new step towards destigmatizing the drug -- and fulfilling a promise to his supporters a month before midterm elections.

"I am announcing a pardon of all prior federal offenses of simple possession of marijuana," Biden said.

He did not call for fully decriminalizing cannabis, saying that "limitations on trafficking, marketing and under-age sales should stay in place."

Instead, the president homed in on individual possession of a substance that the government health authorities estimate was used by at least 18 percent of the population in 2019 -- and which is already permitted by multiple state governments for recreational or medical purposes.

In addition to the pardons, Biden instructed the departments of justice and health to determine whether cannabis should be reclassified as a less dangerous substance.

Officials told reporters that about 6,500 people are directly affected by convictions under federal marijuana statutes. Clemency will extend to thousands more convicted under laws in the federal capital, Washington.

However, Biden's gesture aims to take the shift much further, putting pressure on state authorities everywhere to follow suit.

"I am urging all governors to do the same with regard to state offenses. Just as no one should be in a federal prison solely due to the possession of marijuana, no one should be in a local jail or state prison for that reason, either," Biden said.

- Political impact -
The move was announced abruptly by video message and in a written statement, with no previous build-up by the White House.

However, the impact is expected to be significant, both legally and politically, allowing Biden to seize the narrative on a trend toward decriminalization that swaths of the country have already embraced.

Ahead of the November 8 midterms, where his Democrats are struggling to hold onto even partial control of Congress, Biden has now satisfied a key demand from racial justice activists angered at the way enforcement of cannabis laws often targets ethnic minorities.

"Sending people to prison for possessing marijuana has upended too many lives and incarcerated people for conduct that many states no longer prohibit," Biden said.

He noted that non-white people are disproportionately affected by marijuana possession convictions, which in addition to sometimes including jail time can unleash years of legal fallout, creating difficulties in getting work and education.

The third measure announced was an instruction for federal health and justice officials to "review expeditiously how marijuana is scheduled under federal law."

Currently, federal law lumps marijuana in with what are widely accepted to be far more dangerous narcotics such as heroin and LSD. It is in a group higher than the relatively modern -- and hugely addictive -- drugs fentanyl and methamphetamine.

Senate leader Chuck Schumer, a key Biden ally who is fighting to try and keep the chamber under Democratic control in November, said the president's move recognized that the so-called "war on drugs" has been "a war on people and particularly people of color."

Derrick Johnson, president of the NAACP, a leading Black civil rights organization, said on Twitter: "We applaud President Biden."

"Correcting unequal treatment -- including marijuana reform -- has been a priority issue for the NAACP for decades."

Cynthia Roseberry, with the powerful American Civil Liberties Union, echoed the praise, saying Biden was partly lifting the "long shadow" cast by draconian drug laws, while the Marijuana Policy Project, which campaigns for legal reforms, called Biden's decision "historic."

Republican Senator Tom Cotton, however, accused Biden of going soft on crime in a bid to distract.

"In the midst of a crime wave and on the brink of a recession, Joe Biden is giving blanket pardons to drug offenders -- many of whom pled down from more serious charges. This is a desperate attempt to distract from failed leadership," Cotton said on Twitter.