Qaeda chief Ayman al-Zawahiri accused the US of "plotting" with Egypt's military, secularists and Christians to overthrow president Mohamed Morsi, in an audio recording posted on militant forums.

In his first public comment on the July 3 military coup, the Al-Qaeda boss, himself an Egyptian, said: "Crusaders and secularists and the Americanised army have converged ... with Gulf money and American plotting to topple Mohamed Morsi's government."

In the 15-minute recording, Zawahiri also accused Egypt's Coptic Christian minority of supporting the Islamist president's ouster to attain "a Coptic state stripped from Egypt's south."

Zawahiri attacked Vice President Mohamed ElBaradei, the Nobel Laureate and former UN nuclear watchdog chief who was an opposition leader during Morsi's single year in office.

ElBaradei is the "envoy of American providence," Zawahiri said, labelling the former International Atomic Energy Agency chief as "the destroyer of Iraq."

Zawahiri, who belonged to the militant Egyptian Islamic Jihad group, criticised Morsi's Muslim Brotherhood movement for going soft on applying strict Islamic law.

Morsi's "Muslim Brotherhood government strove to please America and the secularists as much as it could, but they were not satisfied with it," said Zawahiri, who is believed to be hiding somewhere in Afghanistan or Pakistan.

"They did not trust it (Morsi's government) because they did not forget the Brotherhood's slogan: 'Jihad is our war, and death in the path of God is our highest aspiration'," he said.

"The Brotherhood abandoned that slogan, substituting it with the slogan 'Islam is the solution,' but the Crusaders and secularists did not forget," he said.

"What happened is the biggest proof of the failure of democratic means to achieve an Islamic government," he said of the coup.

"I call for them to be united ... to make Islamic law rule."