A U.S. judge expressed skepticism on Thursday toward a bid by President Donald Trump's former campaign chairman to get a raft of charges dismissed, while a prosecutor said Paul Manafort was suspected of being a campaign "back channel" to Russia.

During a hearing, U.S. District Court Judge Amy Berman Jackson questioned Manafort attorney Kevin Downing's assertion that special counsel Robert Mueller overstepped his authority in bringing charges that Downing said fell outside of parameters set by the Justice Department's No. 2 official Rod Rosenstein.

Jackson signaled some willingness to consider whether at least one count in the indictment could be dismissed because the criminal action cited was covered by another charge.