Washington - The United States will send more than 1,000 additional Marines to Afghanistan this month to try to solidify progress in the south before troop reductions begin in July, U.S. military officials said Thursday.

The majority of the forces will be sent to Helmand province, where 20,000 Marines have made gains against the Taliban but where fighting remains intense in insurgent strongholds like Sangin. U.S. commanders are under pressure to quell the violence and sustain their gains in the first six months of this year, when the White House will assess whether a troop increase for the nearly decade-old war is working.

Officials at the Florida-based U.S. Central Command, which has responsibility for the wars in Iraq and Afghanistan, said that the Marines were being sent to take advantage of what is traditionally a winter hiatus for the Taliban and to try to set conditions for the fighting season that begins in the spring.

Maj. Gen. Richard P. Mills, the commander of the 20,000 Marines in Helmand, said in a statement that the intent was to overwhelm the enemy "with an increased operational tempo that he'll be unable to match."

The 1,000-plus Marines are part of the 26th Marine Expeditionary Unit, a reserve force currently deployed in the Indian Ocean and the Arabian Sea. Central Command officials said the Marines would go to Afghanistan for about three months. The rest of the expeditionary unit will remain aboard ship for other contingencies, military officials said.