Some British troops will be serving longer tours of duty as the U.K. prepares to end its combat role in Afghanistan, Britain's defense secretary said Tuesday.

Philip Hammond told lawmakers that Britain's combat role will still close by the end of 2014, but that a brigade deploying in Afghanistan in October will serve eight-month tours. A subsequent brigade could serve a final tour of up to nine months.

Most tours are for six months, but that has been "judged not to be sustainable during the final months of the drawdown," the defense secretary said.

The arrangements — which could affect up to 3,700 troops — will help maintain continuity during the drawdown and the handing over of security responsibilities to Afghan forces. It will also eliminate the need to deploy another brigade.

"The rationale for this decision is clear and is based on advice from military commanders," Hammond told lawmakers.

Britain's role is evolving from engaging in combat to training, advising and assisting Afghan troops. Afghan security forces now lead 80 percent of security operations in most of the country, and Hammond said Tuesday that will be up to 100 percent by the summer.

Britain once had about 9,500 military personnel in Afghanistan. By the end of this year, that number is expected to fall to about 5,200.

Those serving extended tours will get paid extra because they are expected to face "more austere conditions" as bases close and equipment is removed, Hammond said.