Top US Senator John Kerry warned Egypt's military rulers on Tuesday that the planned trial of pro-democracy activists was a "slap in the face" for America that could damage a vital relationship.
Egyptian justices have announced plans to put dozens of pro-democracy activists, including 19 Americans, on trial over alleged illegal funding to foreign aid groups.
The row has led some US lawmakers to openly question the crucial Egypt-US partnership that has anchored America's Middle East policy for a generation and helped keep the peace between Israel and its Arab neighbors.
"I am alarmed by the attacks against civil society in Egypt," Kerry, the influential head of the Senate Foreign Relations Committee, said in prepared remarks in Congress.
"Yesterday's prosecutions are a slap in the face to Americans who have supported Egypt for decades and to Egyptian individuals and NGOs who have put their futures on the line for a more democratic Egypt," said Kerry.
"This is a dangerous game that risks damaging both Egypt's democratic prospects and the US-Egyptian bilateral relationship."
Senator Lindsey Graham, a top Republican on the key senate committee, warned that US assistance to Egypt could be cut if activists end up being imprisoned.
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"If anybody goes to jail I think there'll be a backlash you can't contain," Graham told reporters.
Asked if that meant scrapping $1.3 billion in military aid that Washington provides to its key Arab ally, Graham replied: "Yes, I think very much at risk."
"The red line for me is incarceration. If any American or NGO (non-governmental organization) staff member is pre-trial confined or post-trial confined, that's just an absolute overreaction," he added.
"If they don't really embrace international organizations that are providing assistance to the Egyptian people to help form a new democracy, that's going to be very difficult for the Congress to ignore."
The offices of several local and international NGOs including Freedom House and the International Republican Institute were raided in December by Egyptian authorities as part of a probe into alleged illegal funding.
Then last month, several US members of the NGOs were barred from leaving the country, including Sam LaHood, the son of Transportation Secretary Ray LaHood who is the IRI's country director for Egypt.
American officials said "a handful" of the pro-democracy activists subsequently took refuge inside the US embassy in Cairo, fearing arrest.
Democratic Senator Ben Cardin, a member of the Senate Foreign Relations Committee, said Monday that Washington should "re-evaluate" the bilateral relationship, adding that it was "totally unacceptable" for Egypt to prosecute the activists on charges of illegal funding of aid groups.
The United States on Sunday demanded "clarification" from Egypt over its apparent plans, which threatened to further strain Washington's ties with Cairo's post-Arab Spring military rulers.