Brown said he had "taken responsibility" for shoring up international support for the effort in Afghanistan, saying the "burden-sharing will happen," the BBC reported.
He told the BBC British strategy aligns with that of the United States, where President Barack Obama is considering whether to send more troops to Afghanistan. Brown said the mission focus is strengthening Afghan institutions so the government eventually can take control of its own affairs and "resist the threat of extreme terrorism."
Brown also rejected engaging in talks with the Taliban as a so-called Plan B.
Brown said he thinks Afghan President Hamid Karzai is willing to clean up his government but needs international help to build institutions and give Afghans an economic stake in the country's future.
Brown reportedly ordered Defense Secretary Bob Ainsworth and senior foreign policy adviser Simon McDonald to lobbying other countries to contribute more troops to Afghanistan, The Guardian reported.
"I think we could probably get another 5,000 forces into Afghanistan from that NATO and outside NATO group," Brown said.
However, Liberal Democratic leader Nick Clegg said Britain and the United States had one "last chance" to show they had a plan for success in Afghanistan.
Unless the two countries can develop a credible strategy, Brown "would struggle to look British voters in the eye and say that we should carry on with the war," Clegg wrote in a commentary published Friday in The Times of London.
"There are many people who are beginning to believe withdrawal is the only option," Clegg wrote. "If we fail to adopt a new approach, it inevitably will be."