David Cameron will tell Barack Obama this week that he expects British troops in Afghanistan to pull back from their combat operations by the middle of next year, highlighting the growing pressure on Nato forces to wind down the decade-long conflict.

In a move that provides a tighter deadline for UK withdrawal from Afghanistan, the British premier is expected to say that troops could stop routine patrols in Helmand province within 15 months. UK troops – who form the second-biggest Nato contingent in Afghanistan after the US – would then focus on supporting and training Afghan forces.

Allied strategy is set to dominate the agenda between Mr Cameron and the US president at a two-day summit in Washington this week after an upsurge in violence in Afghanistan. On Sunday, a US soldier killed 16 Afghan civilians in a night-time rampage. The incident, which has placed huge strain on relations between the US and the Afghan population, came days after six UK troops were killed by the Taliban.