Iranian President Hassan Rouhani has accused US-backed Gulf states of carrying out an attack on a military parade that killed 25 people, including soldiers and civilians.
The assault, which wounded at least 70 people, targeted a viewing stand where Iranian officials had gathered in the city of Ahwaz to watch an annual event marking the start of the Islamic Republic's 1980-88 war with Iraq, state television said.
An Iranian ethnic Arab opposition movement called the Ahwaz National Resistance, which seeks a separate state in oil-rich Khuzestan province, claimed responsibility for Saturday's attack.
The Islamic State also claimed responsibility. Neither group provided evidence.
Speaking on Sunday, Rouhani said: "America wants to cause chaos and unrest in our country so that it can return to this country, but these are unreal fantasies and they will never achieve their goals.
The Persian Gulf states are providing monetary, military and political support for these groups- President Hassan Rouhani
"The small puppet countries in the region are backed by America, and the United States is provoking them and giving them the necessary capabilities.
"The Persian Gulf states are providing monetary, military and political support for these groups," said Rouhani
The Iranian president engineered Tehran's 2015 nuclear deal that ushered in a cautious detente with Washington before tensions resurged with President Donald Trump's decision in May to quit the accord and reimpose sanctions.
"America is acting like a bully towards the rest of the world... and thinks it can act based on brute force," said Rouhani.
"But our people will resist and the government is ready to confront America. We will overcome this situation [sanctions] and America will regret choosing the wrong path.
There was no immediate comment from Saudi Arabia on Rouhani's allegations.
"Iran's answer [to this attack] is forthcoming within the framework of law and our national interests," said Rouhani, adding that the United States would regret its "aggressiveness".
Rouhani's allegation will likely ratchet up tensions with Iran's rival Saudi Arabia and its Gulf allies, which along with the US have been working to isolate the Islamic Republic.
Iran and Saudi Arabia back opposing sides in the conflicts in Yemen and Syria as well as rival political groups in Iraq and Lebanon.
'Puppets of the United States'
On Saturday, Iranian Supreme Leader Ayatollah Ali Khamenei ordered security forces to bring to justice those responsible for one of the worst assaults ever against the Revolutionary Guards, several of whom died in the assault.
The Revolutionary Guards are the most powerful military force in the country and answer to Khamenei.
"This crime is a continuation of the plots of the regional states that are puppets of the United States, and their goal is to create insecurity in our dear country," Khamenei said in a statement published on his website.
Women and children died in the assault, in which the four attackers were killed, state news IRNA agency reported.
Women and children died in the assault, in which the four attackers were killed (AP)
Along with Rouhani and Khamenei, Iranian analysts speaking to Middle East Eye (MEE) also accused Saudi Arabia of supporting the attack.
Fereidoun Majlesi, a former diplomat and foreign policy expert, told MEE: "I think the proxy war between Saudi Arabia and Iran is entering a dangerous phase.
"The terrorist attack in Ahwaz is probably supported by Saudi Arabia as the officials in Riyadh have already stated that they would take the war inside the Iran soil.
"The Saudis' plan is wrong. They just united Iran. This is the characteristic of Iranians. When people in Iran face a hardship or an attack, they become united to defeat the invaders.
"I strongly believe that such terrorist actions will only increase the nationalism sentiment."
Speaking to MEE, Rasool Hosseini, a reformist foreign policy analyst, said: "I think today's attack is in line with the US and its regional allies' strategy to pressure Iran.
"I should add that the US secretary of state stated yesterday that they would respond to Iran-backed militias' offensive against US interests.
"The attack occurred one day before Rouhani's trip to New York [to attend the United Nations General Assembly].
"I think Rouhani should now use this opportunity to convey this message to the world that Iran is one of the biggest victims of terrorism, while the US labels Tehran as the state sponsor of terrorism."
Attacks on the military are rare in Iran.
Last year, in the first deadly attack claimed by Islamic State in Tehran, 18 people were killed at the parliament and mausoleum of Ayatollah Ruhollah Khomeini, the founder and first leader of the Islamic Republic.