TEHRAN, Iran—A top aide to Iran’s all-powerful leader has accused the country’s main opposition leader of being an American agent who should be tried for treason, increasing the pressure on reformists disputing the outcome of last month’s presidential election.

There was no immediate reaction from Mir Hossein Mousavi to the accusation in an editorial Saturday in the conservative daily newspaper Khayan. Mousavi, who claims he was fraudulently deprived of victory in the June 12 election that gave Mahmoud Ahmadinejad a second term, has not been seen in public in recent days.

The editorial, by Hossein Shariatmadari, a top aide to supreme leader Ayatollah Ali Khamenei, raised the possibility that Mousavi could be arrested and charged like many other pro-reform figures.

The accusation appeared to be part of an effort by the leadership to portray the opposition and those behind the street unrest as agents working for foreign powers.

A detained Iranian employee of the British Embassy has also

been charged with harming Iran’s national security, his lawyer said Saturday, in a step certain to increase tension with Europe.