US President Donald Trump's former campaign manager Paul Manafort has been reviewing evidence and is being treated like a "VIP" in jail, US prosecutors say, as they argue against Manafort's efforts to postpone his trial on fraud charges.

In a filing to a Virginia federal judge overseeing one of two trials scheduled for Manafort, prosecutors on Wednesday said Manafort's assertions in court papers about conditions in jail, including limited access to evidence and his lawyers, are untrue.

In phone calls taped in his jail, prosecutors said Manafort had remarked that he was being treated like a "VIP", was able to receive daily visits from lawyers and had access to "all my files like I would at home."

Manafort's prosecution spawned from US Special Counsel Robert Mueller's investigation into possible collusion between Russia and Trump's 2016 presidential campaign.

Manafort, who faces charges including bank fraud and failing to register as an agent for pro-Russian politicians in Ukraine, has a trial set for July 25 in Virginia and another trial in September in Washington.

Last week Manafort filed a motion seeking to postpone the Virginia trial until after the Washington trial concludes. His lawyers cited restrictions on emailing and phone calls, and the jail's distance from Washington, as factors that were making it difficult to prepare.

But prosecutors say Manafort had been provided with a personal telephone in his cell, which he used for more than 300 calls with lawyers and others over the past three weeks.

Manafort also appears to have found a workaround to the restriction on electronic communication by reading and composing emails on a second laptop that is carried in and out of the cell by his legal team, the prosecutors said.

"When the team takes the laptop from the jail, it reconnects to the internet and Manafort's emails are transmitted," the prosecutors wrote.

Manafort's lawyers did not respond to a request for comment.

Manafort was remanded to a jail in Warsaw, Virginia about 160 kilometres from Washington following Mueller's allegations that he was tampering with witnesses.