WASHINGTON: Ensuring a long-term US military presence in Afghanistan beyond 2014 will be a key to success or failure in the war against Taliban insurgents, Republican Senator John McCain said Wednesday.

McCain, an outspoken critic of President Barack Obama's planned withdrawal of most US troops from Afghanistan by the end of 2014, said a security accord with Kabul would have a transformative effect on the war.

"The strategic partnership would make clear to the Taliban that it cannot wait us out and win on the battlefield," he said in speech at the Carnegie Endowment for International Peace, a Washington think tank.

US officials say they are optimistic Washington and Kabul will agree a strategic partnership agreement on the future role of American troops in time for a NATO summit on May 20-21 in Chicago.

The accord will likely lay out principles for a security partnership with the details on bases and legal protections to be worked out in a follow-on agreement.

McCain said the deal should allow for US air power, intelligence sharing, access to Afghan bases and support from Special Forces and trainers.

Such an agreement "could change the entire narrative in Afghanistan and the region from one of looming international abandonment to enduring international commitment," he said.

In the meantime, the Obama administration needed to slow down the pace of any drawdown in the next year to preserve battlefield gains and to suspend any plans for scaling back the end-strength of Afghan security forces, he said.

The Afghan army and police are due to reach a maximum of 352,000 troops but US officials are now saying they are planning in the longer-term for a smaller force of 230,00 amid fiscal pressures.

The administration has displayed a "fixation on leaving Afghanistan" that has sent the wrong signal to insurgents and countries in the region, said McCain, who was defeated by Obama in the last presidential election in 2008.