Iranian officials on Tuesday blamed the United States for a last-minute decision by the United Nations to bar Tehran’s participation in a peace conference on Syria, and denied that Iran had agreed to preconditions for attending the talks.

“The U.S. put pressure on [U.N. Secretary General] Ban Ki-moon, and he was forced to cancel this invitation, which shows this international body is still under the influence of big powers, headed by the U.S.,” said Allaedin Boroujerdi, head of the National Security and Foreign Policy Commission of Iran’s parliament.

Ban’s last-minute decision to invite Iran, the primary military backer of Syrian President Bashar al-Assad and a key Middle East power broker, infuriated the Syrian opposition and was strongly opposed by U.S. Secretary of State John F. Kerry, who personally lobbied the U.N. chief to change his mind.

After Ban revoked the invitation on Monday, the main Syrian opposition bloc announced that it would join the long-delayed talks, which are set to begin Wednesday.

The United States opposed Iranian participation unless Tehran endorsed ground rules set in 2012, which state that the goal of the peace talks is a transitional government in Syria, established by the mutual consent of the Assad government and political opponents.

Ban’s spokesman said the U.N. chief was “deeply disappointed” by statements — made by Iranian officials after the invitation was issued — that Iran would not adhere to that framework.

"Given that it has chosen to remain outside that basic understanding, [Ban] has decided that the one-day Montreux gathering will proceed without Iran's participation," Ban spokesman Martin Nesirky said.

Iranian Foreign Minister Mohammad Javad Zarif, who was visiting Turkmenistan’s capital Asghabat, said he had been explicitly clear about Iran’s position.

“During the last week, Ban Ki-moon contacted me several times and I clearly told him that we would not accept any precondition to attend the Geneva conference,” Zarif said.

Foreign ministry spokeswoman Marzieh Afkham said Iran had never asked to participate in the conference in the first place.

“We’re sorry that the Secretary General withdrew his invitation, which was a personal initiative, under pressure, and we hope that he would explain the real reason for the withdrawal,” Afkham said.

Iran has provided Assad’s regime with financial and material support throughout its three-year conflict with opposition groups, in which an estimated 130,00 people have been killed.

Officials in Tehran continue to insist that there will be no solution to the Syrian crisis without their involvement.

Russia, Assad’s other chief ally, had pushed to include Iran at the conference, saying its participation was essential to forging an agreement. The about-face by the United Nation’s about-face was “unseemly,” Foreign Minister Sergei Lavrov said at a news conference Tuesday, according to the Associated Press.

Lavrov said excluding Iran is not a “catastrophe,” and said Russia and the other countries at the conference will still push for a productive dialogue between the warring factions.

But he warned that spurning Iran would deepen division lines in the Muslim world and could negatively impact anti-terrorism efforts, the wire service reported.

The rescinded invitation is an awkward reminder that Iran still has many differences with much of the rest of the world, despite an interim nuclear deal that took effect on Monday.

Under the terms of the agreement, Iran suspended its most sensitive nuclear development work, and world powers lifted some of the sanctions that have crippled the oil-based economy.

Both Zarif and Iran’s president, Hassan Rouhani, are expected to speak at the World Economic Forum in Davos, Switzerland this week.