US Secretary of State John Kerry said on Thursday that NATO was building a "robust" defensive posture on its eastern flank, as the alliance faces an increasingly aggressive Russia.

Speaking after the first day of ministerial talks at the Alantic alliance's Brussels headquarters, Kerry urged member states to meet pledges to increase defence spending.

"A key aspect of our response to Russia's actions in Ukraine is to meet the Wales commitments on defence investments and to continue to strengthen our deterrence capabilities through a more robust forward presence," he said.

In 2014, NATO allies met at a summit in Wales and agreed to halt defence budget cuts and aim to spend two percent of their GDP on their militaries.

Kerry said the ministers would discuss the strategy to defend the bloc's eastern borders later in the day, but that all were agreed that Russia's actions can not stand.

"NATO is open to a political dialogue with Russia but we will refrain from business as usual until the Minsk commitments are fully implemented," he said.

Under the Minsk process, Moscow agreed to a ceasefire in Ukraine and to halt support for separatist pro-Russian rebels that have carved out an enclave in the east of the country.

"Make no mistake, we will not recognise the annexation and the occupation of Crimea," Kerry added.