Kerry arrives for NATO talks on Ukraine, Iraq

US Secretary of State John Kerry arrived in Brussels Tuesday flying in from a visit to Iraq to attend NATO talks on a militant assault which has sparked global alarm.

With attention already on crises in Ukraine and Syria, the Sunni jihadist offensive in northern Iraq has added to a packed NATO agenda with Afghanistan also a high priority.

His trip to meet Iraqi leaders in Baghdad on Monday, followed by Kurdish officials in the autonomous northern region early Tuesday, had been shrouded in secrecy and unusually the top US diplomat arrived in Belgium on the US military plane he had used for security reasons.

Kerry was due to first have talks with EU foreign policy chief Catherine Ashton, before attending 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 the troops to leave in the country, with a top US official predicting a total force of around 12,000.

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

The ministers would also 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, which the West accuses Moscow of fomenting. And Russian Foreign Minister Sergei Lavrov is not attending.

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

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.

"There'll be a lot of effort to coordinate our messages vis-a-vis Russia and coordinate our actions if Russia chooses not to de-escalate" the situation, the US official added.