The head of NATO has pledged to help Ukraine fend off Russia's invasion for "as long as it takes."

Secretary General Jens Stoltenberg spoke to reporters on Friday ahead of a meeting of NATO foreign ministers in Romania next week.

He called on member states to continue providing air defense systems and other weapons to Ukraine.

Stoltenberg pointed out that NATO countries have been delivering fuel, medical supplies, winter equipment and drone jammers, adding that he will "call for more" in Bucharest.

He also stressed the need to help Ukraine "transition from Soviet-era equipment to modern NATO standards, doctrine and training" over the longer term.