BRUSSELS, Sept 9 (Reuters) - NATO Secretary-General Jens Stoltenberg urged allies on Friday to supply Kyiv with winter gear such as clothing, tents and generators to enable Ukrainian troops to keep on fighting Russia's invasion in the cold season.

Average winter temperatures are below freezing for much of the country and it is not unusual for temperatures to drop to minus 15 degrees Celsius.

"The winter is coming, it's going to be hard, and therefore we need both to continue to supply weapons and ammunition but also winter clothing, tents, generators and all the specific equipment which is needed for the winter," Stoltenberg told reporters after meeting U.S. Secretary of State Antony Blinken in Brussels.

"Partly because the size of the Ukrainian army has just increased so much, they need more of this kind of winter equipment, and NATO is particularly focused on how we can provide tens of thousands of, for instance, winter uniforms," Stoltenberg added.

On Friday, swiftly advancing Ukrainian troops were approaching the main railway supplying Russian forces in the east of the country, after the collapse of a section of Russia's front line caused the most dramatic shift in the war's momentum since its early weeks.

It was the first lightning advance of its kind reported by either side for months, in a war mainly characterised by grinding frontline battles since Russia abandoned its ill-fated assault on the capital Kyiv in March.

Stoltenberg called on NATO allies to ramp up defence production, as well to stock up their own inventories so they could deliver more weapons to Kyiv.

"We are now in close contact with the defence industry, with capitals ... to ensure that we are now ramping up production, that we are replenishing the stocks," he said.

"This is also about ensuring that we have the weapons, the ammunitions, the capabilities in place for our own deterrence and defence," he underlined.