Iran's supreme leader, Ayatollah Ali Khamenei, on Monday blamed the United States and the "wicked" British government for creating the Islamic State in his first speech since undergoing prostate surgery last month.
The sharp remarks were a reminder of Iranian suspicions about the West despite the emergence of the ultra-hardline Sunni militants in Iraq and Syria as the common foe of Tehran and Washington.
"America, Zionism, and especially the veteran expert of spreading divisions - the wicked government of Britain - have sharply increased their efforts of creating divisions between the Sunnis and Shi'ites," he said, according to his website, in a speech marking a Shi'ite Muslim religious holiday.
Islamic State, known to its detractors by its Arabic acronym Da'esh, has overrun swathes of war-torn Syria and Iraq in recent months.
Despite being adversaries for decades, Shi'ite power Iran and the United States both oppose the militants and have armed local groups fighting them. Senior officials from both countries have denied any plans to work together, however.
"They created Al Qaeda and Da'esh in order to create divisions and to fight against the Islamic Republic, but today, they have turned on them (Islamic State)," Khamenei said.
The United States along with several Sunni Arab monarchies began a campaign of air strikes against Islamic State militants in Syria on September 23.
Other Western countries, including Britain, have also taken part in bombing raids against Islamic State positions in Iraq.
Khamenei's accusation appeared to be reference to Western support for the rebel forces fighting Tehran's close ally, Syrian President Bashar-al Assad. Hardline Islamists have emerged as the rebels' strongest military element.
Iran also believes the United States and Britain are using the Islamist threat to justify their renewed presence in the region.
"A careful and analytic look at the developments reveals that the US and its allies, in efforts that are falsely termed countering Daesh, seek to create division and enmity among the Muslims rather to destroy the root causes of that (terrorist) current," Khamenei said.
"Shi'ites and Sunnis must know that any action or remark, including insulting one another, leads to increased sensitivities and ignite flames. This will certainly benefit the common enemy of all Muslims."
Khamenei's criticism was a counterpoint to an apparent thaw in British-Iranian relations when President Hassan Rouhani met British Prime Minister David Cameron in New York in September - a move that was criticised by hardliners at home.
That meeting followed decades of strained relations which worsened when Britain closed its embassy in Tehran after hardliners stormed it in November 2011.
Britain decided in June this year to reopen the facility, but the embassy has yet to open its doors.