By David Brunnstrom

WASHINGTON (Reuters) - An international conference is expected to pledge over $3 billion a year in development support for Afghanistan next week but funds will be dependent on reforms and countering corruption, a senior U.S. official said on Thursday.

Richard Olson, U.S. special representative for Afghanistan and Pakistan, told a Washington forum the U.S. government would seek via Congress to maintain U.S. assistance "at, or near" current levels for the period to 2020.

The European Union and Afghanistan will host a donor conference on Oct. 4-5 in Brussels to seek backing for reforms to stabilize and develop the country. Some 70 states and 30 international organizations and agencies will attend.

The conference comes in the context of a greatly scaled-back U.S. and international military presence in Afghanistan, and is aimed at helping the Kabul government establish an agenda for increased self-reliance.

Olson said he expected the Afghan government to announce "ambitious" medium-term plans to wean itself of donor support and stimulate economic growth.

"We expect strong pledges of renewed support to be announced next week, collectively totaling over $3 billion a year in development support through 2020," he said.

Olson said U.S. Secretary of State John Kerry would be "ready to indicate clearly American intentions to maintain a strong leadership role within the broad coalition of allies and partners engaged in Afghanistan."

Olson stressed the assistance was "not a blank check" and would be dependent on Afghan progress in carrying out reforms, including countering corruption.

U.S. senators warned this month that failure to address corruption could cause them to rethink the billions of dollars the United States spends in Afghanistan each year.

A Sept. 14 report from the U.S. Special Inspector General for Afghanistan Reconstruction strongly criticizing Washington for pouring billions of dollars into Afghanistan with so little oversight that it fueled corruption and undermined the U.S. mission.

The United States spends $5 billion a year in Afghanistan, - about $4 billion for defense and national security and another $1 billion in civilian assistance, plus billions more for thousands of U.S. troops and military contractors there.

Olson said the goal was to strengthen Afghan security forces and institutions to allow them to gradually defeat the Taliban insurgency. He said he hoped bipartisan consensus on this would be maintained under the next U.S. administration after the Nov. 8 U.S. election.

Earlier this month, India promised $1 billion in aid to Afghanistan during a visit to New Delhi by Afghan President Ashraf Ghani.

(Reporting by David Brunnstrom; Editing by Diane Craft and Paul Simao)