U.S. President Donald Trump stepped up his criticism on Monday of former Democratic President Barack Obama's handling of alleged Russian interference in the 2016 U.S. presidential campaign, accusing his predecessor of collusion and demanding an apology from investigators.

Trump has increasingly lashed out over the ongoing investigations into Russia's role in the election and possible collusion by his campaign. The Republican president said Obama "colluded or obstructed," but gave no other details about his accusation.

"The reason that President Obama did NOTHING about Russia after being notified by the CIA of meddling is that he expected Clinton would win ... and did not want to 'rock the boat.' He didn't 'choke,' he colluded or obstructed, and it did the Dems and Crooked Hillary no good," he wrote, referring to Democrats and Hillary Clinton, his rival in the presidential election.