At least five assailants in the deadly Tehran attacks were recruited by the Islamic State from inside Iran, the government said Thursday, a strong indication they were Iranian citizens.

The new detail about the assailants, who were killed during the attacks on Wednesday, came as the Iranian news media reported that the civilian casualty toll had risen to 17 dead and 52 wounded, and as the police presence in the Iranian capital increased noticeably.

The assaults’ aftermath also was punctuated by new acrimony between Iranian leaders and the Trump administration, which expressed condolences on Wednesday coupled with an assertion that their country had fallen “victim to the evil they promote.” The assaults also coincided with Senate action advancing new sanctions on Iran.

Foreign Minister Mohammad Javad Zarif described the American response in a Twitter message as “repugnant.”

Five men and a woman armed with rifles and explosives carried out simultaneous assaults on Tehran’s Parliament building and the tomb of Iran’s revolutionary founder in broad daylight, surprising security forces and causing panic and mayhem that lasted for hours. The Islamic State, the Sunni extremist group, asserted responsibility.

Official Iranian news accounts said the men were killed and the woman blew herself up.

It was Iran’s worst episode of terrorism in years, exposing security lapses and undermining government assertions that the country is a beacon of calm in the volatile Middle East.

The attacks also appeared to be the first time that Iran, a predominantly Shiite Muslim nation, had been successfully targeted by the Islamic State, which considers Shiites to be religious traitors.

A government statement issued Thursday about the attacks said the male assailants had left Iran at an unspecified time to fight for the Islamic State in Mosul, Iraq, and in Raqqa, Syria, the group’s de facto capital.

They returned to Iran last July or August under the leadership of a commander with the nom de guerre Abu Aisha, the statement said, and had “intended to carry out terrorist operations in religious cities.”

The statement did not specify whether they were Iranian citizens or provide further information about the female assailant.

But Reza Seifollahi, deputy chief of the Supreme National Security Council, was quoted by the independent newspaper Shargh as saying the men were Iranian.

If true, that would be an unusual acknowledgment, given the antipathy between the Islamic State and Iran. Most of Iran’s 80 million people are Shiites, although sizable Sunni minorities inhabit some border regions and the Islamic State has sought to recruit from among them.

In March, the Islamic State released a video featuring Iranian fighters, in which it called on Sunnis in Iran to form cells and attack Shiite forces, according to the SITE Intelligence Group, which analyzed the video.

Several witnesses to the attacks reported that the assailants had spoken Arabic with an Iranian accent, suggesting that they were ethnic Arabs living in Iran.

In Khuzestan, an oil-rich province that borders Iraq and that is home to many Arabs, both Sunni and Shiite, a video emerged two weeks ago of men in black carrying weapons and shouting slogans on the streets of Ahvaz, the provincial capital. They were arrested the next day, the Intelligence Ministry said.

A southeastern province, Sistan and Baluchistan, is home to several extremist Sunni groups that have committed bombings, assassinations and other attacks on Iranian security forces and officials in recent times. Iran’s intelligence minister, Mahmoud Alavi, said on Thursday that Iran had broken up “a hundred terrorist plots” over the past two years, according to the news site Asr-e Iran. Former inmates of Evin Prison in Tehran have said they saw dozens of incarcerated Sunni extremists, often Kurds and Baluchis. Several of them have been hanged.

Mr. Alavi suggested the assailants killed on Wednesday had affiliations with the ultraconservative form of Sunni Islam practiced in Saudi Arabia, Iran’s regional rival, but stopped short of directly blaming the Saudis.

“There is this belief in the world that Saudi Arabia is the ideological source of these terrorist movements, but it is too soon to say Saudi Arabia was behind the attack because we don’t want to make statements without evidence,” Mr. Alavi was quoted by the Iranian Students’ News Agency as saying during a visit to victims at a Tehran hospital.

Statements of support for Iran continued to pour in on Thursday.

Prime Minister Haider al-Abadi of Iraq expressed his condolences in a message to President Hassan Rouhani.

President Bashar al-Assad of Syria, who has received crucial support from Iran during his country’s six-year-old civil war, spoke by telephone with Mr. Rouhani and affirmed his determination to fight “terrorists and their supporters,” according to a report from Syrian state media. Both Iran and Syria portray the war as a conflict against Western- and gulf-sponsored terrorism.

The Lebanese militant group Hezbollah, which serves as a proxy for Iran in Syria and Lebanon, also condemned the attacks, calling them part of an “international, destructive plan.”