A senior US senator has moved to block hundreds of millions of dollars in aid to Egypt to protest the death sentences imposed on nearly 700 suspected Islamists.

Denouncing Egypt's "dictatorship run amok" and its "egregious violation of human rights," Democrat Patrick Leahy, who heads the Senate subcommittee overseeing foreign aid appropriations, said he is putting a hold on the $650 million in military aid greenlighted by the Pentagon last week.

"I am not prepared to sign off on the delivery of additional aid for the Egyptian military," Leahy said in a floor speech Tuesday.

"I am not prepared to do that until we see convincing evidence the government is committed to the rule of law."

The US administration last week partially lifted a six-month freeze on some $1.5 billion in mostly military aid to Cairo -- a key regional ally that has relied for decades on American aid.

Washington had agreed to deliver 10 Apache helicopters for counterterrorism efforts in the unruly Sinai peninsula and $650 million in military aid, but withheld the rest of the funding until democratic progress is made.

But Leahy said Egypt is not worthy of the assistance until it takes immediate steps to improve its rights record.

"We cannot stand here and say we are troubled by hundreds of people being sentenced to death after a few minutes in a mass trial, but since we have been friends for so long we will go ahead and send you hundreds of millions of dollars in aid. No."

Leahy added: "I do not think the taxpayers of this country would condone that, and neither do I."

Several US lawmakers in recent months, including prospective 2016 Republican presidential candidate Senator Rand Paul, have questioned the Obama administration's aid to Cairo in the aftermath of last year's overthrow of democratically-elected president Mohamed Morsi.

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Leahy's action came as Egyptian Foreign Minister Nabil Fahmy was visiting Washington, the highest-level Egyptian official to do so since the army overthrew Morsi.

Fahmy met with Secretary of State John Kerry on Tuesday, and the minister is scheduled to meet Wednesday with leaders of the Senate Foreign Relations Committee.

That panel's chairman, Senator Robert Menendez, told AFP the death sentences imposed by an Egyptian court "are clearly terribly alarming, and make us wonder where Egypt is headed."

He said he would discuss the aid matter with Fahmy.

The minister appeared on US television Wednesday to allay concerns about his government, saying "we will correct mistakes if they occur."

But he warned of moving too swiftly to block aid.

"If you use that (aid) simply as a tool to -- as leverage against every argument or disagreement you have with Egypt -- you'll end up... questioning and shaking the very roots of the strategic relationship," Fahmy told MSNBC.

"We will do better, not because you want us to do better, because Egyptians want us to do better."

Blocking aid would no doubt prick the US administration and potentially force it into complicated maneuvering to honor its Egypt commitments.

A Leahy aide said that continuing payments on existing US contracts is not the issue.

In the complex and longstanding US aid-funding process, new assistance is presented to congressional committee, and if no members object within 15 days, the aid moves ahead.

A block like Leahy's normally triggers consultations with the administration, which would work with lawmakers to reach an agreement before the aid flows.