WASHINGTON (AFP) - The United States is "deeply concerned" about the death sentences handed out against 529 supporters of Egypt's deposed president Mohamed Mursi, a US official said on Monday.

An Egyptian court handed down the sentences earlier on Monday, amid a sweeping crackdown on supporters of the Islamist president, who was overthrown by the army last July.

"While appeals are possible, it simply does not seem possible that a fair review of evidence and testimony consistent with international standards could be accomplished with over 529 defendants after a two-day trial," a State Department official said.

The largest mass sentencing in the country's history was related to the death of a policeman and other violence in the aftermath of the clearing of two squares last August.

"We continue to call on the Egyptian government to ensure that all those detained in Egypt are afforded fair proceedings that respect civil liberties and due process and are consistent with international standards. The law must be applied equitably and free of political bias," the State Department official said.

"We have said many times that even the appearance of politically-motivated arrests, detentions, and convictions will set Egypt's transition back," the official added.

The official, citing reports of violence in response to the sentences, warned that such a reaction would not help either.

The United States partially suspended its US$1.5 billion (S$1.9 billion) in annual aid to long-time ally Egypt, much of it to the military, after last summer's crackdown on the opposition.

Since then, it has regularly criticized the new interim government for the slow transition to democracy and its poor record on human rights and public liberties.

But the US administration has never declared Morsi's overthrow a coup, and even US Secretary of State John Kerry has contended that the army acted as it did to save democracy.

He also recently suggested there would be a decision soon to resume US aid.