BRUSSELS - NATO will step up fighting in Afghanistan next year as 37,000 more troops join the conflict, and is aware that increased civilian casualties may be the result, a military spokesman said on Wednesday.

The United States is sending 30,000 more troops to the war, and NATO allies this week promised a further 6,800 from 36 countries, lifting the total foreign force in Afghanistan to about 140,000 once all are on the ground next year.

The extra troops will help train Afghan police and soldiers, so that Afghans can eventually take over responsibility for security. But they will also allow U.S. and NATO combat units to intensify fighting against the resurgent Taliban.

I think the intent of General McChrystal with the force strength is as soon as possible to demonstrate the ability to do just that, Brigadier-General Eric Tremblay, spokesman for the NATO-led International Security Assistance Force in Afghanistan, said on Wednesday when asked if combat would intensify.

McChrystal is commander of U.S. and NATO forces in Afghanistan.

With the arrival of additional forces you can certainly expect that some areas will still need to be cleared, held and built -- there will be some areas where those additional forces will conduct their missions.

In the past during the eight-year conflict, an increase in combat operations, frequently accompanied by air strikes, has led to more civilian deaths, causing friction between the Afghan government and military leaders.

McChrystal has made avoiding civilian deaths a central aim in all operations. But Tremblay said increased fighting generally risked more casualties.

An increase in tempo could lead to increased collateral damages, he said, adding that the risk would be mitigated by following tactical directives McChrystal has issued.

It's all about leadership and to ensure that people understand that we are there to protect the Afghans, and leadership from top-to-bottom to ensure that we minimize collateral damages, he said.

The increase in combat operations is expected to start in the spring of next year, when Afghanistan's snows have melted and the Taliban-led insurgency usually resumes its attacks.

U.S. General David Petraeus, the head of U.S. Central Command and McChrystal's boss, warned on Wednesday that violence in Afghanistan may increase in the short term and asked U.S. lawmakers to wait at least a year for results.

Achieving progress in Afghanistan will be hard and the progress there likely will be slower in developing than was the progress achieved in Iraq, he told Congress, adding Afghanistan was likely to get harder before it gets easier.

(Reporting by Luke Baker)