The US State Department and the British foreign secretary have condemned the "terrorist attacks against the Egyptian armed forces in North Sinai on Friday."
Two attacks by unknown assailants against Egyptian soldiers in the restive peninsula left tens of soldiers killed or injured.
The US pledged its continued support, as part of the strategic partnership between the two countries, for Egyptian government efforts to counter the threat of terrorism in Egypt, a US State Department communique said.
Meanwhile, British Foreign Secretary Philip Hammond denounced the attacks, highlighting "UK support to the Egyptian government in their fight against terrorism."
Speaking Friday night, Hammond said: "I will be offering my personal condolences to Foreign Minister Sameh Shoukri when we meet on Monday in London and look forward to discussing practical cooperation to tackle terrorism during our talks."
Some 28 soldiers were killed and another 30 injured when a car bomb exploded at the Karm Alkwadis security checkpoint in Sheikh Zuweid in North Sinai. Hours later three security personnel died when militants opened fire on a checkpoint in nearby Al-Arish.
Over 40 security personnel have been killed in attacks in the Sinai Peninsula this week, including the attacks Friday. A militant insurgency by jihadist groups in the peninsula has become more active since the ouster of former Islamist president Mohamed Morsi in 2013. Hundreds of police and soldiers, as well as militants, have been killed as a result.
Egypt declared a state of emergency in parts of North Sinai starting Saturday morning, accompanied by an overnight curfew. Egypt also declared three days of mourning.