US lawmakers on Sunday were split over whether to cut military aid to Egypt, a key regional ally, after its violent crackdown on supporters of ousted Islamist president Mohamed Morsi.

While condemning the use of force by the military-backed interim government, many lawmakers expressed concern that halting aid would further erode US influence over the most populous Arab country.

US Senator John McCain, who called for suspending the $1.3 billion in annual aid to the military after it overthrew Morsi in early July, said Washington risked losing credibility if it continued to turn a blind eye to the bloody crackdown.

"They have orchestrated a massacre," he said, after the four-day death toll from mass shootings and street clashes climbed to more than 750 people.

"We have no credibility. We do have influence, but when you don't use that influence, then you do not have that influence," McCain, a Republican hawk and frequent critic of President Barack Obama's foreign policy, told CNN's "State of the Union."

McCain suggested Washington could pressure Egypt's generals by cutting off aid, spare parts for US-made military equipment and backing for an International Monetary Fund loan to relieve the country's devastated economy.

"For us to sit by and watch this happen is a violation of everything that we stand for," he said.

Senator Lindsey Graham, another prominent Republican who visited Egypt with McCain in a failed bid to resolve the crisis earlier this month, also called for suspending aid.

Graham warned that the Islamist opposition could soon be transformed into an armed insurgency, calling Egypt an "absolute disaster in the making."

"We're headed for Algeria," he told CBS's Face the Nation, referring to the civil war in the 1990s that erupted after the military canceled elections ahead of an expected Islamist victory.

Morsi's Muslim Brotherhood supporters "will go underground, Al-Qaeda will come to their aid and you'll have an armed insurgency, not protesters, on your hands in the next 60 days or 90 days, and we'll have a failed state in Egypt," Graham warned.

Obama last week canceled joint military exercises but has yet to suspend aid to Egypt, a key Middle East ally for decades and one of just two Arab countries to have signed a peace treaty with Israel.

The administration has refused to call Morsi's overthrow a "coup," which would legally require it to cut off aid, saying it hoped to steer the country toward a democratic transition.

Several lawmakers have expressed concern that cutting off aid could endanger the peace treaty with Israel or compromise US privileges with regard to the Suez Canal.

Others have pointed out that most of the aid to Egypt for this year has already been dispersed, saying they expect a debate over next year's tranche in the coming months.

Democratic Senator Richard Blumenthal said the administration should "condition our future aid on specific steps toward the rule of law and the return to democracy."

"We shouldn't cut off all aid. There are no good choices here. But of the two, there is more opportunity to protect American interests if we work with the military," he said on "Fox News Sunday."

Democratic Representative Eliot Engel, appearing on ABC News's "This Week," said Washington shouldn't "throw the baby out with the bath water."

"Egypt's an important country, and I think we have to be very careful before we willy-nilly just cut off aid."

The military has defended its overthrow of Morsi, the country's first democratically elected president, as a response to massive protests against his rule. It has blamed the violence on Morsi's Islamist backers, whom it refers to as "terrorists."

Egypt's military chief Abdel Fattah al-Sisi showed no signs of backing down Sunday despite the heightened US pressure, saying security forces would confront further violence from protesters.

"We will never be silent in the face of the destruction of the country," Sisi told top military and police commanders, pledging a "forceful" response to further attacks on police stations and government buildings.

Morsi's supporters meanwhile said they had canceled several marches, citing fears of vigilantes and snipers, but that other protests would go ahead.