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.
The United States, which has close ties to the Egyptian military, said it was "very concerned" by Sisi's call and decided to suspend a plan to supply the country with F-16 warplanes.
Sisi made his unprecedented move in a speech broadcast live on state television.
"Next Friday, all honourable Egyptians must take to the street to give me a mandate and command to end terrorism and violence," said the general, wearing dark sunglasses as he addressed a military graduation ceremony near Alexandria.
A spokesman for army-installed interim president Adly Mansour later said Egypt "has begun a war on terrorism."
Sisi's call for protests was aimed at "preserving the state," said spokesman Ahmed al-Muslimani, in comments carried by the official MENA news agency.
A coalition of Islamists led by Morsi's Muslim Brotherhood said it would press ahead with its 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 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 one 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.
His address came just hours after a police conscript died when a time bomb exploded outside 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 two soldiers in separate attacks in the Sinai peninsula, where militants have launched daily attacks on security forces since Morsi's overthrow.
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At least three militants also died in the Sinai when their booby trapped car apparently exploded before they reached their intended target, said MENA.
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 wrote on Facebook.
He was referring to demonstrations by Morsi's supporters which have continued non-stop since the military ousted the Islamist leader on July 3 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 to support 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.
Morsi's detention, and the subsequent arrests of senior Brotherhood leaders, have hardened his supporters' stance.
His daughter Shaimaa Mohamed Morsi said this week that the family would sue Sisi and also take legal action outside Egypt.
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 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.
State Department spokeswoman Jen Psaki said Washington was "very concerned about the calls" made by Sisi for protests.
"We're concerned about the possibility of this leading to more violence," she said.
Psaki said the United States remained focused on "encouraging the interim government to move towards an inclusive process, which includes elections."