US President Joe Biden's son, Hunter Biden, appeared at a federal court on Wednesday, where he is on trial for tax evasion.

During the hearing he pleaded not guilty to two tax charges after a plea deal with federal prosecutors fell apart.

The younger Biden had been expected to plead guilty  in order to avert conviction on a charge related to gun possession in a separate case.

CNN and Bloomberg reported that the original plea deal unraveled after a federal judge expressed concern over the gun charge and uncertainty about whether the agreement gave Hunter Biden protection from future charges.

What are the charges against Hunter Biden?

Hunter Biden is accused of not paying taxes on more than $1.35 million (€1.22 million) in income in 2017 and 2018, with prosecutors saying that he owed more than $100,000.

In a separate case, he is charged with unlawfully owning a firearm while addicted to and using a controlled substance. This is classified as a felony under US law.

Under a plea deal agreed with federal prosecutors in Delaware, Hunter Biden agreed to enter into a pretrial diversion agreement for that charge. This is an alternative to prosecution that allows defendants to avoid conviction and prison time.

Prosecutors had planned to recommend two years of probation. The charges carry a maximum sentence of 10 years in prison.

US District Court Judge Maryellen Noreika said she was concerned about the language in the plea deal and recommended lawyers discuss it.

"I think having you guys talk makes sense," she said.

Hunter Biden's attorneys told the court that the plea agreement was null and void, the CNN broadcaster reported.

Republicans accuse Hunter Biden of leveraging political power

Besides the three charges leveled against him, Republicans have for years accused Hunter Biden of leveraging his father's political power for gains in dealings with Ukraine and China.

A probe by US Attorney David Weiss did not present any evidence to support the claims.

Republicans have also alleged that the plea deal reached between Hunter Biden and federal prosecutors constituted an example of favorable treatment.

sdi/nm (Reuters, AP, AFP)