Iran's Supreme Leader Ayatollah Ali Khamenei has said nuclear talks with world powers would continue, though there was no point in holding talks with the United States on other issues.

The country's most senior religious leader said there was no benefit in negotiating with the US except in certain cases, a statement published on his website said on Wednesday.

"Of course, in the area of nuclear talks, we will not stop them," his website quoted him as saying.

Khamenei, who has final say over all state matters in Iran and was addressing foreign ministry officials when the comments were made, criticised the US, saying direct talks with its diplomats did not help reduce sanctions or decrease its animosity towards Tehran, the AP news agency reported.

"Some pretended that if we sit down with Americans at the negotiating table, many of the problems will be resolved,'' Iranian state television quoted Khamenei as saying.

Iran and the six world powers it is negotiating with over its nuclear programme, including the US, agreed to a four-month extension of negotiations. 

Shared concerns

After decades of frought relations between the country and western powers, diplomatic relations have thawed with the election of President Hassan Rouhani last year.

Iran shares similar concerns as the West over Iraq, where much of the country's west and north have collapsed in the face of an offensive by Sunni rebels led by the Islamic State group.

Khameini joined the US and other Western powers in praising the appointment of Haider al-Abadi as Iraq's new prime minister.

"I hope the designation of the new prime minister in Iraq will untie the knot...and teach a good lesson to those who aim for sedition in Iraq," Khamenei said according to the Reuters news agency.

The statement is seen as the strongest sign yet that Tehran was no longer standing by its old ally Nouri al-Maliki.