KABUL (Reuters) - The United States is aiming to send 20,000 to 30,000 extra troops to Afghanistan by the beginning of next summer, the chairman of the U.S. Joint Chiefs of Staff said on Saturday.

U.S. Defense Secretary Robert Gates poses with a marine after speaking with troops at Kandahar Air field, December 11, 2008. GREUTERS/Scott Olson/Pool

Washington is already sending some 3,000 extra troops in January and another 2,800 by spring, but officials previously have said the number would be made up to 20,000 in the next 12 to 18 months, once approved by the U.S. administration.

“Some 20 to 30,000 is the window of overall increase from where we are right now. I don’t have an exact number,” Admiral Mike Mullen told reporters in Kabul.

“We’ve agreed on the requirement and so it’s really clear to me we’re going to fill that requirement so it’s not a matter of if, but when,” he said.

“We’re looking to get them here in the spring, but certainly by the beginning of summer at the latest.”

U.S. Army General David McKiernan, the commander of international forces in Afghanistan, has asked for the extra troops to combat a growing Taliban insurgency in the east and south of Afghanistan.

U.S. President-elect Barack Obama has pledged a renewed focus on Afghanistan, where U.S.-led forces toppled the Taliban government in late 2001 after the September 11 attacks.

The United States now has some 31,000 troops in Afghanistan.

After the January deployment, most of the reinforcements are to be sent to southern Afghanistan to bolster mainly British, Canadian and Dutch troops who have suffered heavy casualties in the last two years fighting in the Taliban heartland.

“That’s where the toughest fight is,” Mullen said. “When we get additional troops here, I think the violence level is going to go up. The fight will be tougher.”

He said beefing up U.S. forces in Afghanistan was linked to winding down in Iraq.

“Available forces are directly tied to forces in Iraq. As we look to the possibility of reducing forces in Iraq over the course of the next year, the availability of forces to come here in Afghanistan will increase,” Mullen said.

INDIA-PAKISTAN

Mullen said the attacks by Islamist militants in Mumbai last month showed the need to reduce Indian tensions with Pakistan and that would help bring stability to Afghanistan.

“That’s another big piece of the strategy, what I would call regional focus to include Pakistan, Afghanistan and India ... leadership in all three of those countries to figure out a way to decrease tensions, not increase tensions,” Mullen said.

He said the late arrival of winter this year had meant there were still significant flows of militants from the tribal belt along the Pakistani side of the border, but better cooperation with the Pakistani military was nevertheless helping.

“We’re not there, we still have a long way to go but we’ve actually made a lot of progress,” Mullen said.

Mullen said the Afghan government was not as strong as he had anticipated and engaging with tribal areas in remote parts of Afghanistan could be central to future operations.

“We may have overstated the focus on the ability of the central government to have the kind of impact that we wanted given the history here in Afghanistan,” Mullen said.

Mullen also said at the same time, more must be done to boost economic development in Afghanistan, one of the world’s poorest countries, and to make the Afghan government more effective.

“No amount of troops, no amount of time will provide a solution here without development,” he said.