Neither Iran's Hassan Rouhani nor Russia's Vladimir Putin feature on President Barack Obama's "dance card" of meetings with foreign leaders at the U.N. next week, the White House said Monday.

Speculation about possible talks between Obama and Rouhani in particular has been mounting because the counterparts spoke by phone last year on the sidelines of the annual United Nations General Assembly.

Appearing Sunday in Iowa, a critical state in the U.S. presidential election cycle, Hillary Clinton joked about her potential 2016 White House bid, telling Democratic supporters she was "thinking about it."

The head of the main Syrian opposition group in exile said Sunday he expected the support of the international community to press the fight against Damascus and Islamic State jihadists.

The United States is pushing for the formation of a broad international coalition to tackle IS which has captured swathes of Iraq and Syria, and has already secured the backing of 10 Arab states including Saudi Arabia.

The Islamic State claimed the beheading of a British aid worker on Saturday, an act slammed as "pure evil" by Prime Minister David Cameron who vowed Britain would do all it could to catch the killers.

President Barack Obama offered U.S. support for its "ally in grief", while Cameron faced growing calls to allow Britain's military to help in Washington's planned assault against the rampaging jihadist group.

With momentum building for international military action against Islamist extremists in Syria, the United States reiterated Friday it will not coordinate with strongman Bashar Assad in the effort.

The White House declared Friday the United States was at war with Islamic State radicals, seeking to rub out another semantic flap over its Syria policy.

The jihadists who have declared a Muslim "caliphate" in parts of Syria and Iraq call themselves the Islamic State but detractors say they represent neither Islam nor any state.

In a speech outlining his strategy against the group on Wednesday, President Barack Obama pointedly denied the extremists their chosen appellation.

Syria must be part of the coalition against jihadists because the country is a key victim of "terrorism", an adviser to President Bashar Assad said on Friday.

"Terrorism didn't start in Syria today, it started four years ago" when the uprising against Assad began, Buthaina Shaaban was quoted as saying by state news agency SANA.

A man jumped the perimeter fence of the White House on Thursday, jangling nerves on the 13th anniversary of the September 11 attacks.

Foreign Minister Jebran Bassil stressed Thursday during talks in Jeddah the importance of not “excluding any country or party” that is willing to counter the Islamic State and asked for military assistance to the Lebanese Army.

“There shouldn't be any exclusion of countries or parties that are willing to counter IS, since there is no reason or justification for such a rejection,” said Bassil during a meeting between U.S. Secretary of State John Kerry and his Arab and Turkish counterparts aimed at building an anti-IS coalition.