Egypt's army chief called Wednesday for rallies to back a crackdown on "terrorism and violence", in comments Islamists denounced as a call to "civil war" ahead of their own protests.

With tensions already running high three weeks after the military ousted president Mohamed Morsi, General Abdel Fattah al-Sisi's call for demonstrations raises the prospect of further deadly violence on the streets.

And reflecting its concerns about the instability rocking Egypt, the United States said it had decided to suspend a plan to supply the country with F-16 fighter jets.

"Next Friday, all honourable Egyptians must take to the street to give me a mandate and command to end terrorism and violence," said Sisi, wearing dark sunglasses as he took to the podium to address a graduation ceremony of military cadets near Alexandria.

A coalition of Islamists led by Morsi's Muslim Brotherhood said they would press ahead with their own rallies on Friday.

"Sisi's threats are an announcement of civil war," the group said, while warning of the danger of "massacres committed under a false popular cover".

Nearly 170 people have died in political unrest in Egypt since the end of June, according to an AFP tally, many of them in clashes between Morsi's supporters and opponents.

Huge crowds of Egyptians protested against Morsi on June 30, after just a single turbulent year of his presidency.

Sisi claimed he had been told by Morsi aides that removing the president would result in violence.

Presidential aides "told me if there is any problem there will be lots of violence because of armed groups, to scare me," Sisi said in his speech.

Sisi's address came just hours after a police conscript died when a time bomb exploded in front a police station in Mansura in the Nile Delta, state television and the interior ministry said.

After Sisi's speech, security sources said gunmen killed a soldier in the Sinai peninsula, where militants have carried out daily attacks on security forces since Morsi's overthrow.

At least three militants also died in the Sinai when their booby trapped car apparently exploded before they reached their intended target, said the official MENA news agency.

Senior Brotherhood leader Essam al-Erian said Morsi loyalists would not be intimidated by the army chief's call for mass rallies.

"Your threat will not prevent millions from continuously protesting," Erian said on Facebook.

He was referring to demonstrations by Morsi's supporters which have continued non-stop since the military ousted the Islamist leader and placed him in custody.

Tamarod, the movement that spearheaded the mass anti-Morsi rallies that led up to the coup, called on supporters to take to the streets again on Friday in support of the army.

"We call on the great Egyptian people to rally on Friday across Egypt to demand... Morsi's trial and to support the military in its upcoming war on terrorism."

Clashes between supporters and opponents of Morsi have killed at least 13 people across Egypt since Monday.

Presidential spokesman Ahmed Al-Muslimani denounced the violence, saying "Egypt is not a second Syria and anyone who pushes in that direction is a traitor".

But Morsi's detention, and subsequent arrests of senior Brotherhood leaders, have hardened his supporters.

His daughter Shaimaa Mohamed Morsi said this week that the family would sue army chief General Sisi and also take legal action outside Egypt.

Senior aides held with the deposed president have asked their families for prison clothes, signalling possible charges, relatives said.

The United States has joined other Western nations in calling for Morsi's release, although it has declined to characterise his overthrow as a coup, which would force a suspension of US aid.

But the Pentagon said Defence Secretary Chuck Hagel on Wednesday informed Sisi of the US decision to delay the planned delivery of an additional four F-16 fighter jets to Egypt.

"Given the current situation in Egypt we do not believe it is appropriate to move forward at this time with the delivery of F-16s," spokesman George Little told reporters.

"We remain committed to the US-Egypt defence relationship as it remains a foundation of our broader strategic partnership with Egypt and serves as a pillar of regional stability," he added.

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