Looking for His glorious appearing! Titus 2:13

U.S. Secretary of State John Kerry refused be drawn into a "back and forth" with Iran on Monday following Tehran's assertion that it had rebuffed a U.S. request to coordinate a response to the Islamic State jihadist group.

"I'm not going to get into a back and forth. I don't want to do that. I don't think that is constructive, frankly," Kerry told reporters at the U.S. ambassador's residence in Paris after an international conference on Iraq attended by 26 countries.

Earlier, Iran's supreme leader, Ayatollah Ali Khamenei, said Tehran had vetoed a U.S. invitation to cooperate in fighting Islamic State. "The American ambassador in Iraq asked our ambassador (in Iraq) for a session to discuss coordinating a fight against Daesh (Islamic State)," said Khamenei, in quotes carried on state news agency IRNA.

Asked if that news was false, Kerry replied, "I have no idea of what interpretation they drew from any discussion that may or may not have taken place. We are not coordinating with Iran. Period."

He said he hoped Tehran and Washington could find common ground in the next round of nuclear peace talks, which begin this week.

"I don't express levels of optimism on it. I'm hopeful that it will be possible to find a way to reach an agreement that is important to the world but there is some very difficult issues," he said.

Foreign ministry officials from six world powers - the United States, Britain, France, Germany, Russia and China - will hold their first full negotiating round with Iran since July on Thursday in New York, seeking to narrow gaps over the future size of Iran's uranium enrichment infrastructure and other issues.

Depending on how those talks progress, they could move to the level of foreign ministers on the sidelines of the U.N. General Assembly session the week of Sept. 22, U.N. diplomats say.

Iran denies Western allegations it is refining uranium to develop the capability to assemble nuclear weapons, saying it is doing it to help generate electricity. The United States and its allies have in recent years imposed ever tighter financial and others sanctions on Iran, a major oil producer, to make it scale back its nuclear program.

Diplomats say the main stumbling block is disagreement on how many centrifuges Iran should be allowed to keep to refine uranium, with Tehran rejecting demands to significantly reduce the number below the more than 19,000 it has now installed, of which roughly half are operating.

Earlier on Monday, world powers backed military measures on Monday to help defeat Islamic State fighters in Iraq, boosting Washington's efforts to set up a coalition, but made no mention of the tougher diplomatic challenge next door in Syria.