The top US commander in Afghanistan said Thursday he favored keeping major "combat power" in place in 2013 with 68,000 American troops on the ground, despite political calls for a faster exit.

With the US force due to be reduced from nearly 90,000 to 68,000 by the end of September, General John Allen was pressed for his views on troop drawdown plans after the summer and indicated he preferred no fresh withdrawals beyond those already planned.

"My opinion is that we will need significant combat power in 2013," he told the Senate Armed Services Committee.

When asked by Senator John McCain how many troops would be required next year, Allen said: "Sixty-eight thousand is a good going-in number sir, but I owe the president some analysis on that."

His comments suggested military leaders are reluctant to speed a planned troop withdrawal from Afghanistan amid a debate inside the White House and Congress about the pace of the drawdown.

A series of damaging setbacks in Afghanistan, including the burning of Korans at a US base that sparked violent unrest, has fueled demands in Washington to pick up the pace of the drawdown as Afghan forces gradually take the lead.

Pressure is building in particular from members of President Barack Obama's party, with fellow Democrats convinced the war effort is producing diminishing returns while Afghan President Hamid Karzai has urged an accelerated exit of foreign troops.

McCain, a hawk on the war in Afghanistan, said discussion of withdrawal plans was undermining the mission and sending the wrong signal to allies, to Taliban insurgents and to Pakistan.

"We don't hear anymore commitments to victory, we don't hear anymore commitments to success and it shouldn't surprise you or anybody, general, when President (Hamid) Karzai exhibits some of the behavior that he does, that the Taliban feel that they can wait us out, that the Pakistanis continue to support the Haqqani network and continue to hedge their bets.

"Because, all they hear about is withdrawals and pace of withdrawal."

Under NATO's strategy, Afghan army and police are due to take over security for the whole of the country by the end of 2014, while the United States hopes to keep a small force in place post-2014 pending negotiations with the Kabul government.

Allen also acknowledged that the Pentagon was looking at dramatically scaling back the size of the Afghan army and police after 2014, dropping a target for a 352,000-strong force.

The security forces are scheduled to reach 352,000 this year but a study just completed suggests an end-strength of only 230,000 by 2017, the general said.

An Afghan army and police force of 230,000 was "the right target given what we think will be the potential enemy scenario for 2017," Allen said.

A final decision would be based on security conditions and not financial considerations, Allen said, though NATO governments are anxious about the cost of funding the Afghan troops.

Subsidizing a smaller 230,000-strong force would cost Western countries about $4.1 billion a year, officials said.

Afghan President Hamid Karzai cited the $4 billion figure Thursday in remarks in Kabul on Thursday, implicitly accepting a cut in the planned size of his government's military.