CAIRO — The dominant Islamic militant group in Egypt pledged its allegiance to the organization that calls itself the Islamic State on Monday, becoming its first international affiliate and expanding the challenge to the government in Cairo.
The announcement, made in an audio statement posted online, is a milestone and a lift to morale for the Islamic State six weeks into a US-led bombing campaign on its strongholds in Syria and Iraq.
The Egyptian group, Ansar Beit al-Maqdis, is the leading militant group in the most populous Arab state. Its decision is the latest manifestation of a swirling descent into violence around the region amid the dashed hopes for democracy of the Arab Spring uprisings three years ago.
Later Monday, militant groups in Libya and Yemen pledged their loyalty to the Islamic State in online messages, the Associated Press reported. The separate pledges carried no known names of militant groups, instead referring generally to the ‘‘holy warriors’’ in the two countries.
Among the Libyan groups pledging loyalty to the Islamic state was the one in control of the eastern city of Derna, which has long been known as Libya’s center for jihadi radicals.
Until Monday, few prominent jihadist thinkers or factions had endorsed the Islamic State’s grandiose claims to leadership of all jihadists and even all Muslims.
Ansar Beit al-Maqdis rose to prominence in the northern Sinai Peninsula 16 months ago in a violent backlash against the military ouster of President Mohammed Morsi.
It has faced an unusual crackdown from the new military-led government, recently including the forced evacuation of more than 1,100 families from an area near the border with Gaza that has been a haven for the group.
The new alliance is a gamble that the Islamic State can help the Egyptian militants survive and expand by providing money, weapons, and recruits.
The move carries the risk that the Islamic State’s reputation for indiscriminate violence will alienate other Egyptians, especially the disaffected Islamist youths that Ansar Beit al-Maqdis has sought to enlist in its fight, according to Western officials familiar with intelligence reports on the group’s internal communications.
Unlike the Islamic State, Ansar Beit al-Maqdis has focused its attacks almost exclusively on military and security forces, and Egyptian and Western officials worry that the new alliance could lead the group to target Christians and tourists.
The group has already carried out at least a handful of beheadings of people suspected of being government informants in Sinai, adopting the signature punishment of the Islamic State. The announcement of the affiliation alone may unnerve investors and tourists.
In its audio statement, Ansar Beit al-Maqdis pledged to “obey” Abu Bakr al-Baghdadi, the Islamic State’s leader and self-proclaimed caliph.
“To our people in Egypt, what are you waiting for after the violation of your dignity?” the statement said, reiterating a call to arms that has been a hallmark of the group. “After shedding the blood of your sons on the hands of this reckless tyrant and his soldiers?
“When will you take out your swords to face your enemies?” it continued, adding, “Shameful peace will do you no good, nor will blasphemous democracy, and you have seen how it has claimed its upholders and their masters.”
Details about Ansar Beit al-Maqdis are scarce, in part because the Egyptian military limits access to the areas of Sinai that are the front line of its battle against the group.
The group’s name means Supporters of Jerusalem, and in the 16 months since Morsi’s ouster, it has exceeded the expectations of most Western officials as well as the Egyptian government’s confident predictions. Western officials estimate that the group includes perhaps a few thousand fighters.
It has recruited skilled and experienced fighters and staged increasingly sophisticated and deadly attacks on military camps across the country — including one in the Western desert July 19 that killed at least 21 soldiers and one Oct. 24 in Sinai that killed at least 31.
Western officials say Ansar Beit al-Maqdis includes cells of fighters scattered on both sides of the Nile. Sympathetic Islamist militants to the east and west of the country — in lawless eastern Libya on one side, and in the Palestinian territory of Gaza on the other — have taken advantage of porous borders to provide refuge and supplies.