The presidents have plenty to discuss. In addition to Ukraine, Obama plans to press Putin on Russia’s support of Syrian President Bashar al-Assad, and he’ll assess Russia’s potential as an ally against the self-proclaimed Islamic State.

The USA has agreed to talk with Russian Federation about “deconflicting” their military action in Syria. Russian Federation and Iran have provided crucial support to Assad since Syria’s uprising began in 2011.

While the Foreign Affairs Minister Charlie Flanagan will attend United Nations summits in the USA this week.

But on Sunday he told reporters President Assad had “butchered his own people” and was “one of the great recruiting sergeants” for so-called Islamic State.

Poroshenko stressed that Ukraine and its US and European allies need to be steadfast with Russia to make sure it lives up to the terms of the agreement to resolve the differences in eastern Ukraine, where pro-Russian separatists with support from Moscow have seized territory in the Donbass region and battled government forces.

A spokesperson said President Obama will be asking President Putin just why he is enhancing his military involvement Syria.

He said that his country will continue to work closely with the U.S.-led coalition that has been bombing the IS group in Syria and Iraq, saying Iraq needs “all the world’s intelligence efforts in order to fight, to besiege and to terminate Daesh”. “But what we do have is a significant coalition of countries that are seized with this threat and that are enhancing their capabilities, and that are joining with us in this effort”, Rhodes said, noting that Iraq’s Prime Minister Haider al-Abadi would join the session.

“There are more than 2,000 militants in Syria from the former Soviet Union”, he added.

Cameron’s change of tone comes as he is under pressure to demonstrate he has a coherent strategy for Syria before asking Parliament to vote on extending Britain’s air strikes on ISIS in to Syria.

Obama and Putin have long had a strained relationship and their body language in face-to-face meetings is always closely scrutinized for signs of tension.

The president will address the UN General Assembly Monday, where he is expected to talk about this and other issues. There has been much debate over how the Obama administration has handled ISIS, with some conservatives in Congress now calling for America to take the lead by sending in troops.

At a later meeting between the two leaders on the sidelines of the United Nations event, Obama is likely to want to focus also on the situation in Ukraine, where Kyiv, Washington and North Atlantic Treaty Organisation accuse Moscow of backing and supplying a pro-Russian insurgency in the east of the country.