A man charged with plotting to fly remote-controlled model planes packed with explosives into the Pentagon and US Capitol plans to plead guilty to two charges, his lawyers and prosecutors said in a plea agreement filed in federal court.

Rezwan Ferdaus, a Muslim-American with a physics degree from Boston's Northeastern University, was arrested in September after federal employees posing as al Qaida members delivered materials he had allegedly requested, including grenades, machine guns and what he believed was 11 kilograms of C-4 explosives.

In the plea agreement, prosecutors and his lawyers say Ferdaus will plead guilty to attempting to provide material support to terrorists and attempting to damage and destroy federal buildings by means of an explosive.

Prosecutors and defence lawyers have agreed to request a 17-year sentence.

Under the agreement, prosecutors have agreed to dismiss four other charges.

Authorities said the public was never in danger from the explosives, which they said were always under the control of federal officials during the sting operation.

Counter-terrorism experts and model-aircraft enthusiasts said it would be nearly impossible to inflict large-scale damage of the kind Ferdaus allegedly envisioned using model planes because the aircraft are too small, cannot carry enough explosives and are too difficult to fly.

Authorities alleged that Ferdaus, 27, became convinced that America was evil. He allegedly contacted a federal informant and later began meeting to discuss the plot with undercover agents he believed were members of al Qaida.

He was charged with planning to use three remote-controlled aeroplanes, each packed with five pounds of explosives, to blow up the Pentagon and US Capitol.

At one point, Ferdaus allegedly told undercover agents that his desire to attack the United States was so strong "I just can't stop. There is no other choice for me", according to a recorded conversation detailed in an affidavit filed in court.

Ferdaus's lawyers have suggested that the FBI ignored signs of mental illness while investigating him.

During a bail hearing in November, an FBI agent acknowledged that the bureau had received reports about bizarre behaviour by Ferdaus, including a report to police about one incident in which he allegedly stood in the road not moving and appeared to have wet his pants.