The US and its allies will retain a presence in Afghanistan big enough to bolster Afghan forces after the withdrawal of international combat troops at the end of 2014, the recently retired commander in Afghanistan, General John Allen, said on Monday.

Speaking in Washington, Allen said he had never been asked to produce a report on the so-called "zero option" – the suggestion that no American troops would remain after the 2014 deadline, floated by one White House adviser in January.

Instead, Allen said that he expected that Obama would approve a force that would be commensurate with ensuring that the Afghan security forces could be properly supported.

Obama is currently considering how many troops are to be left behind, mostly in an advisory capacity, after the official withdrawal in 2014.

Speculation on the size of the force ranges from about 6,000 through to 20,000. Allen offered Obama various options about force size before retiring last month. He ruled out a full pullout, an option the White House deputy national security adviser Ben Rhodes had said in January was on the table.

"I was never asked to conduct any analysis with respect to the zero option," Allen told a meeting at the Brookings Institution in Washington on Monday.

Relations between the US and the Afghanistan president Hamid Karzai have been strained since he accused Washington two weeks ago of colluding with the Taliban, prompting questions in America about why it bothered to keep any troops in the country.

Allen rejected Karzai's accusation outright. "If the president truly does believe the US is colluding with the Taliban, I am here to tell you [that] I would know – and it ain't so."

In spite of the complexity of the relationship between the Obama administration and Karzai, whose term is due to finish next year, Allen stressed that the US and other countries intended to stick with Afghanistan for the long term.

"Sometimes this comes as a surprise when I say this: that on January 1 2015, there's still going to be fighting in Afghanistan," Allen said.

After 2014, the main mission would be to build up the Afghanistan army and police, Allen said. But he did not want to pre-empt Obama on the number of US and international forces needed to achieve that.

"The numbers I have seen can accomplish those objectives. I believe we will be right on the edge of the resources, but I think we will get the resources that will be necessary to accomplish those missions," he said. The aim is to maintain an Afghan force of about 352,000.

The Afghan army has been widely criticised, with frequent predictions that it will fall apart in the face of the Taliban insurgency without the backing of US and international forces. But Allen said the criticism should not be directed at ordinary Afghan soldiers who were among the first into the line of fire. The problem had been with commanders and officers and there had been a surprising improvement throughout all ranks.

The Afghan security force, he said, "turned out to be better than we thought, and they turned out to be better than they thought". © Guardian News and Media 2013