The Islamic State is urging jihadists in the United States and Europe to carry out terror attacks during what a group spokesman promised would be "the month of conquest and jihad."

A U.S. government advisory warned that ISIS attacks, either directly sponsored or simply inspired by the group’s rhetoric, could be carried out during Islam’s month of Ramadan that begins June 5.

The threatened attacks were announced in an ISIS video made public May 21.

In the video, ISIS spokesman Abu Mohammad al-Adnani called on jihadists to "get prepared, be ready … to make it a month of calamity everywhere for nonbelievers…especially for the fighters and supporters of the caliphate in Europe and America."

A report by the State Department-led Overseas Security Advisory Council said the threat could be credible because three deadly terror attacks took place last year shortly after Adnani issued a similar call to arms.

"According to Islamic practice, sacrifice during Ramadan can be considered more valuable than that made at other times, so a call to martyrdom during the month may hold a special allure to some," the report said.

There is no information indicating a specific ISIS attack is being planned, although the threat of attacks remains significant, the report said.

The report, produced by the State Department’s diplomatic security office, said there is no data to indicate Ramadan witnesses increased terrorism, "but as [ISIS] remains focused on expanding conflict, it is likely to continue to seek to carry out attacks during the holy month."

Ramadan includes a period of fasting for Muslims and is held each year during the ninth month of the Islamic lunar calendar.

ISIS declared its so-called Islamic caliphate during Ramadan in 2014.

After the broadcast provocation from ISIS, the State Department urged American businesses operating abroad to "remain aware of the persistent threat of [ISIS] attacks, both inspired and directed."

Adnani, in the video, did not say what targets should be attacked but stated that "the smallest action in their heartland is better and more." He also urged jihadists to conduct lone-wolf attacks and said doing so would produce "the great reward of martyrdom."

"The terrorism risk at the moment is generally high," the report said.

Attacks merely inspired by the group pose a special threat because of the inability to track and counter jihadists not linked directly to ISIS, headquarted in Syria.

"The lone-wolf threat is less likely to be bound by geographic boundaries, which may make it less difficult to identify higher-risk locations," the report said.

The Department of Homeland Security has not raised the terrorism alert level so far in response to the potential threat. The State Department has not issued a new warning to American travelers, although it updated a travel warning to those visiting Europe that terror attacks could be carried out at the European soccer championship scheduled to begin June 10, and a Catholic World Youth Day in Poland July 26.

The State Department’s annual report on terrorism, made public this week, called ISIS "the greatest threat globally, maintaining a formidable force in Iraq and Syria, including a large number of foreign terrorist fighters."

Although the group has lost territory as a result of international counterterrorism operations, ISIS is expanding in Libya and other parts of the world.

"Beyond affiliated groups, [ISIS] was able to inspire attacks in 2015 by individuals or small groups of self-radicalized individuals in several cities around the world," the report said, noting that the group’s propaganda and social media efforts have challenged counterterrorism activities.

A CIA spokesman declined to comment on the Ramadan threat. State Department and Department of Homeland Security spokesmen did not respond to requests for comment.

The report said ISIS in the Middle East is "likely to continue attacks during Ramadan, and could seek to take advantage of some aspects of the holy month—for example, targeting security forces during Iftar (the evening meal when Muslims break their daily fast during Ramadan)."

"But attacks are also likely to continue beyond Ramadan," the report noted. "Lone-wolf extremists could also carry out attacks in response to the call."

In Europe, ISIS could exploit the summer tourist season and sporting and other events for high profile attacks. "Terrorists wishing to maximize the impact of an attack and increase civilian casualties may view this as a favorable timeframe for symbolic attacks against soft targets in Europe, including major events, tourist sites, restaurants, commercial centers, and transportation," the report said.

For coordinated attacks similar to those carried out in Paris and Brussels, the required planning could make attacks timed to Ramadan difficult although the timeframe provides "a larger window of opportunity," the report said.

"Calls for Ramadan attacks by [ISIS] may inspire unaffiliated lone wolves who generally require less time for planning," the report said. "While Ramadan could be an added impetus for terrorists to carry out attacks against perceived enemies in Europe, the threats are unlikely to stop when the month comes to a close."

In Africa, the Islamist terror group Boko Haram inflicted heavy casualties during 2015 attacks during Ramadan in northeastern Nigeria. The Somalia-based al-Shabaab also conducted attacks in the past during the religious month.

In Afghanistan, the Taliban also is expected to step up attacks in the coming month, notably to demonstrate that the killing of its leader Mullah Mansour in a U.S. drone strike in Pakistan, has not diminished its capabilities.

ISIS also could use the Ramadan period to announce new affiliates in Bangladesh and elsewhere. It is not expected to conduct attacks in Southeast Asia or in South America during the next month.