US Secretary of State John Kerry huddled with European allies ahead of key NATO talks Wednesday, after a whirlwind visit to Iraq aimed at shoring up Iraqi unity.

Shortly after flying in on a US military plane from northern Iraq, Kerry met late Tuesday with EU foreign policy chief Catherine Ashton as well as other European partners and "discussed the grave security situation in Iraq."

They also talked about "efforts to de-escalate tensions in Ukraine and efforts to support the political process in Libya," State Department spokeswoman Jen Psaki said.

With crises boiling over in Ukraine and Syria, the Sunni jihadist offensive in northern Iraq has added new urgency to an already packed NATO agenda, with ministers also due to discuss efforts to wind down the war in Afghanistan.

"As everybody knows this is a very critical time for Iraq," Kerry warned in Arbil Tuesday as he met with Kurdish leaders to discuss the onslaught by the Islamic State of Iraq and the Levant (ISIL).

His talks came as the first of up to 300 US military advisers began their mission in Baghdad to help the Iraqi army, which has wilted in face of the militants' seizure of a swathe of northern Iraq.

Kerry briefed European allies from Germany, France, Italy and Britain on his trip "to Baghdad and Arbil and expressed strong concern about the threat posed by ISIL," Psaki said.

He also attended a dinner of foreign ministers from the 28-member alliance.

US President Barack Obama announced earlier this month that the US will scale back its troops in Afghanistan to 9,800 before withdrawing them completely by the end of 2016.

That will shape plans by other NATO members who have forces in Afghanistan on how many troops to leave in the country, with a top US official predicting a total force of around 12,000.

- 'Very focused' on Ukraine -

But NATO's formal meetings, which open Wednesday, are set to be "very, very focused on the situation in Ukraine," the official told reporters.

The ministers would discuss the implementation of new Ukrainian President Petro Poroshenko's peace plan, "the ceasefire efforts, and also efforts to negotiate with eastern players," the official said, asking not to be named.

The White House on Tuesday welcomed Russian President Vladimir Putin's call to his lawmakers to revoke his authorisation to invade Ukraine.

White House spokesman Josh Earnest however said Washington, which has threatened further sanctions on Moscow, wanted to see clear evidence of a change in Russian behaviour.

The usual NATO-Russia meeting has been cancelled because of the unrest in eastern Ukraine, blamed on Moscow by the West. And Russian Foreign Minister Sergei Lavrov is not attending.

Instead Kerry will have his first bilateral meeting with new Ukrainian Foreign Minister Pavlo Klimkin.

Ukraine is also asking for extra support from NATO as it seeks to quell the pro-Russia separatists who have seized control of parts of eastern Ukraine.

"Ukraine has traditionally been one of NATO's most active partners. It's contributed to every single NATO mission. And now Ukraine is asking for allied support as it seeks to rebuild its own security forces in the wake of events," the US official told reporters.

Kerry will Thursday hold back-to-back meetings with Gulf allies in Paris to brief them on his talks in Iraq and discuss the bloody three-year war in Syria.

He will meet Saudi Foreign Minister Saud al-Faisal, UAE top diplomat Abdullah bin Zayed and his Jordanian counterpart Nasser Judeh, a second senior State Department official said.

The top US diplomat will also hold talks with Israeli Avigdor Lieberman to be briefed about the kidnapping of three Israeli teenagers in the West Bank nearly two weeks ago.