WASHINGTON: The chairman of the House of Representatives Armed Services Committee called on the Obama administration on Monday to ask Congress for more money to fund its plans for Iraq and Afghanistan, amid expectations of a defence budget battle when lawmakers return to Washington this fall.

"The administration needs to come over with a supplemental (budget request)," Republican Representative Mac Thornberry said at a roundtable discussion with reporters shortly after returning from a trip to Iraq and Afghanistan.

He said US and allied forces were making progress in Afghanistan and in the fight against Islamic State militants in Iraq, but that stiff challenges remained in both campaigns.

Thornberry and other congressional Republicans have been saying President Barack Obama must formally request additional funds for the fiscal year that starts on Oct. 1 since he announced plans earlier this month to send more troops to Iraq and keep more in Afghanistan than had been expected.

The request for more troops, without a plan to pay for them, added another wrinkle to the simmering dispute between the Republican-led Congress and the Democratic administration over spending. Obama has threatened to veto a $602 billion defence policy bill because of its use of special funds for the wars in Afghanistan and Iraq to avoid mandatory budget limits.

Democrats say the Department of Defence should be subject to the same spending restrictions as nonmilitary programs. Republicans accuse Democrats of stinting on national security in order to fund pet domestic projects.

"That is holding our military hostage," Thornberry said.

The congressman said he had had no official numbers from the Pentagon but that he had been told it faces roughly $6 billion more in expenses than when the administration made its budget request earlier this year.

He said military commanders in Afghanistan were grappling with how to handle Obama's plan to reduce the number of troops in Afghanistan to 8,448 by December 31 from about 9,800 currently.

They are looking into using more contractors and whether some functions can be moved out of Afghanistan, Thornberry said.

Defence Secretary Ash Carter has said the Pentagon was evaluating how to proceed, comparing its estimated costs with what is already in its budgets.