CAIRO, Egypt -- Gunmen killed three policemen at a security checkpoint north of Cairo on Monday, the Interior Ministry said as suspected Islamic militants waging an insurgency in the Sinai Peninsula increasingly spread their campaign to mainland Egypt.

Egyptian security forces have been battling a full-fledged insurgency by the militants -- including several al Qaeda-linked groups -- in the volatile northern part of Sinai since shortly after the July ouster of Mohammed Morsi, an Islamist who became Egypt's first freely elected president in June last year.

The militants' campaign has been mostly confined to the troubled peninsula, which is separated from the mainland by the Suez Canal, but attacks outside Sinai have grown in frequency in recent weeks.

The violence has included an assassination attempt on the interior minister last month, a car bombing of the military intelligence building in the Suez Canal city of Ismailia earlier this month, and the killing of four Coptic Christians and a Muslim outside a Cairo church last week.

No group immediately claimed responsibility for Monday's killings in the city of Mansoura, but such attacks are typical for militants opposed to Egypt's military-backed government.

Morsi was deposed in a July 3 coup after mass protests calling for him to go. His ouster was followed by a large-scale crackdown on his Muslim Brotherhood group in which hundreds have been killed and at least 2,000 members jailed.

Morsi's supporters, in the meantime, have been staging near-daily protests across much of Egypt to demand his reinstatement. The number of participants in these protests has steadily dwindled, although they occasionally attract thousands.

Several hundred university students loyal to Morsi protested on Monday in an eastern Cairo district near the site where security forces on Aug. 14 stormed a pro-Morsi sit-in encampment known as Rabaah al-Adawiyah, killing hundreds.

As the protest got underway, army troops backed by armoured vehicles blocked off the road leading to the site, creating a tense stand-off.

Morsi, detained and held at an undisclosed location since July 3, is to go on trial on Nov. 4 for allegedly inciting supporters to kill protesters outside his presidential palace in Cairo last December.

The harsh crackdown, Morsi's upcoming trial and the street protests have further divided Egypt, roiled in turmoil and violence since the ouster in Feb. 2011 of longtime ruler Hosni Mubarak. Some of the liberals who backed Morsi's ouster are now unhappy with what they see as a throwback to the human rights violations and the police brutality of the Mubarak era.

The liberals are up against a much larger camp that vehemently supports the army and police, saying the troops' excesses are a small price to pay if the country is to defeat what they view as Brotherhood-inspired terrorism.