US Defense Secretary Robert Gates on Sunday called for harsher economic penalties against Iran, as he believes that such sanctions are more likely to bring success to US efforts to pressure Teheran into halting its nuclear program than diplomacy. Economic penalties are more likely to get the Iranians to the table, the defense secretary said on "Fox Sunday News." If enough economic pressure is placed on Iran, diplomacy might provide "an open door" for Iranians if they choose to change their policies, Gates explained. Gates went on to say that Teheran lacks the capability at this point to enrich enough uranium to the levels needed for a weapon. While the US defense secretary called for economic pressure on Iran, one of the country's top diplomats, Richard Holbrooke, expressed pessimism regarding a possible warming of relations with the Islamic republic in the near future. Holebrooke, the US envoy to Afghanistan and Pakistan said on Sunday that he was not getting his hopes up for a breakthrough with Iran during an upcoming international conference in the Netherlands. Thirty years of bitter disagreements between the US and Iran won't be erased in one meeting, said Holbrooke. Holbrooke appeared on CNN's "State of the Union." US Secretary of State Hillary Rodham Clinton and Iranian diplomats will be together at The Hague, in the Netherlands, for a UN-sponsored meeting Tuesday on Afghanistan.