The Islamic State has been planning an imminent attack on subway systems in the US and Paris, Iraq's prime minister has warned.

Prime Minister Haider al-Abadi said the bomb plot has not yet been thwarted, although the United States has been alerted.

Mr al-Adabi made the announcement on the sidelines of the United Nations General Assembly in New York.

He told a small group of US reporters: "Today, while I am here, I am receiving accurate reports from Baghdad where there was arrest of few elements and there are networks planning from inside Iraq to have attacks."

"They plan to have attacks in the metros of Paris and the US," he added. "From the details I have received, yes it looks credible."

There was no immediate verification from the US and French authorities.

An American-led coalition launched airstrikes inside Syria on Monday, expanding weeks of attacks by the US on Islamic State targets in Iraq.

Speaking at the Assembly earlier, Iran's President Hassan Rouhani said the West must take responsibility for strategic foreign policy "blunders" which have contributed to the rise of extremism.

Mr Rouhani told the UN the West had helped create "terror havens" in parts of the Middle East and Central Asia.

"Improper interference in the developments in Syria are clear examples of this erroneous strategic approach in the Middle East," the Iranian President said.

"The strategic blunders of the West in the Middle East, Central Asia and the Caucasus have turned these parts of the world into a haven for terrorists and extremists."

US President Barack Obama called on Wednesday for the world to join together to destroy the group, which he branded a "network of death."

In the latest airstrikes, Saudi Arabia and the United Arab Emirates joined the US-led operation to destroy oil refineries in eastern Syria which produce revenue for IS.

But Syrian Kurds who fled towns and cities overrun by IS said the group had responded to the airstrikes by focusing its assault near Syria's border with Turkey.

Mazlum Bergaden, a teacher from Kobani, said: "The situation is very bad. After they kill people, they are burning the villages. When they capture any village, they behead one person to make everyone else afraid."

Some 140,000 Syrian Kurds have crossed into Turkey in the last four days alone, and authorities are preparing for many more.