Oil prices firmed on Monday on tensions over Iran's nuclear programme but gains were capped by concerns about global economic growth and consequently oil demand. Brent crude futures rose 38 cents, 0.5%, to $64.61 a barrel by 10:45am EDT (2 "45pm GMT). US West Texas Intermediate (WTI) crude futures rose 47 cents, or 0.8%, to $57.98 a barrel. Iran on Monday threatened to restart deactivated centrifuges and step up its enrichment of uranium to 20% in a move that further threatens the 2015 nuclear agreement that Washington abandoned last year. Washington has imposed sanctions that eliminate benefits Iran was meant to receive in return for agreeing to curbs on its nuclear program under the 2015 deal with world powers. The confrontation has brought the US and Iran close to conflict, with US President Donald Trump calling off air strikes last month minutes before impact. On Sunday, Trump issued another warning over Iran's nuclear activities. " They'd better