The attack happened on Friday while the bus was travelling on the road to the St Samuel Monastery in the Minya governorate.
Cairo: Masked militants riding in three SUVs opened fire Friday on a bus packed with Coptic Christians, including children, south of the Egyptian capital, killing at least 28 people and wounding 22, the Interior Ministry said.
No group immediately claimed responsibility for the attack, the fourth to target Christians since December, but it bore the hallmarks of the Islamic State group.
The attack came on the eve of the start of the Muslim holy month of Ramadan.
Islamic militants have for years been waging an insurgency mostly centered in the restive northern part of Egypt's Sinai Peninsula, although a growing number of attacks have recently also taken place on the mainland.
The assault happened while the bus was traveling on a side road in the desert leading to the remote monastery of Saint Samuel the Confessor in Maghagha, in Minya governorate, about 220 kilometers (140 miles) south of Cairo where Christians account for more than 35 percent of the population, the highest ratio in any province.
Security officials quoted witnesses as saying they saw between eight and 10 attackers dressed in military uniforms and wearing masks. The victims were en route from the nearby province of Beni Suef to visit the monastery.
Security and medical officials, speaking on condition of anonymity because they were not authorized to talk to reporters, said the death toll stood at 28 but feared it could rise.
According to Copts United news portal, only three children survived the attack. It was not immediately known how many of the victims were children.
Arab TV stations showed images of a badly damaged bus along a roadside, many of its windows shattered and with numerous bullet holes. Footage of the bus's interior showed blood stains on the seats and shattered glass.
Ambulances are seen parked around the bus and bodies are lying on the ground, covered with black plastic sheets.
Another video purported to show about 10 bodies of mature men lying on the sand on the side of the road with pools of blood around them. Children hysterically screaming could be heard in the background.
The surge in attacks targeting Christians in Egypt has added to the challenges facing President Abdel-Fattah el-Sissi's government as it struggles to contain the insurgency while pushing ahead with an ambitious and politically sensitive reform program to revive the country's ailing economy that already has sent the cost of food and services soaring.
There was no immediate word from the government on how it intends to stop similar attacks from taking place, but it is likely to heighten security around churches, monasteries, schools and annual pilgrimages to remote Christian sites across the country.
On Wednesday, Egypt blocked access to nearly two dozen websites it said were sympathetic to militants or spreading their ideology.
"The growing number of these terror attacks is not at all reassuring," Father Rafic Greiche, spokesman for the Egyptian Catholic church, told a local television station.
El-Sissi called for a meeting with top aides to discuss Friday's attack. Last month, he declared a three-month state of emergency following twin suicide bombings that struck two churches north of Cairo on Palm Sunday.
In December, a suicide bomber targeted a Cairo church. The attacks left at least 75 dead and scores wounded and were claimed by IS, which later vowed more attacks against Christians.
Late last month, Pope Francis visited Egypt, in part to show his support for Christians in this Muslim majority Arab nation following the December and April attacks.
During the trip, Francis paid tribute to the victims of the December bombing in Cairo's St. Peter's church, located in close proximity to Cairo's St. Mark's cathedral, the seat of the Coptic Orthodox Church.
He visited the church, where he joined in prayers the spiritual leader of Egypt's Orthodox Christians, Pope Tawadros II, and leaders of other Christian churches in Egypt.
Following the pope's visit, IS vowed to escalate attacks against Christians, urging Muslims to steer clear of Christian gatherings and Western embassies, saying they are targets for the group's followers.
Egypt's Copts, the Middle East's largest Christian community, have long complained of discrimination, as well as outright attacks, at the hands of the country's majority Muslim population.
They account for about 10 percent of Egypt's 93 million people. More recently, some Christians have complained that the government was not doing enough to protect them.
They rallied behind el-Sissi, a general-turned-president, when he in 2013 ousted his Islamist predecessor Mohammed Morsi, who hails from the Muslim Brotherhood group.
Attacks on Christian homes, businesses and churches subsequently surged, especially in the country's south, the heartland of Egypt's Christians.