The United States on Friday renewed its plea to Egyptian protestors not to incite violence on the streets of Cairo, as it closely monitored renewed clashes in Egypt.

Washington was "troubled by the reports ... of small clashes in Cairo," deputy State Department spokeswoman Marie Harf told reporters.

"We condemn all acts of violence as well as incitement of violence. We've also said the Egyptian government has a responsibility to protect all Egyptians and create an atmosphere that supports a process of political transition that is peaceful, inclusive and has maximum participation."

An Egyptian medical official told AFP that four people were killed in Cairo and 40 injured in Friday's clashes between Islamist supporters of ousted president Mohamed Morsi and security forces.

But Harf also insisted the protestors should ensure that all the demonstrations were peaceful.

The "protesters also have a responsibility to protest peacefully, to not incite violence, to not undertake violence of their own," Harf said.

And she renewed calls for the opposition to take part in political talks with the military-led interim government aimed at restoring democracy.

"Even though it's very tough after what we've been through over these last few months, they have a responsibility to be a part of the process; that's the best thing for the Egyptian people."