By Jeff Mason and Alister Bull
WASHINGTON (Reuters) - President Barack Obama said on Wednesday the window for a diplomatic solution with Iran over its nuclear program was shrinking and he encouraged Tehran to seize the opportunity of talks to avert even worse consequences than current economic sanctions.
Obama, speaking at a news conference with British Prime Minister David Cameron, said Iran's tendency has been to stall talks with world powers.
"We will do everything we can to resolve this diplomatically but ultimately we've got to have somebody on the other side of the table who's taking this seriously and I hope that the Iranian regime understands that," Obama said.
"The window for solving this issue diplomatically is shrinking."
Last month Iran proposed a resumption of long-stalled nuclear talks with world powers -- the United States, Russia, China, France, Britain, and Germany -- known as the "P5+1."
Washington and its allies suspect Iran is seeking to develop nuclear weapons capability under the cover of a declared civilian nuclear energy program. They believe Tehran has used talks only as a time-buying tool instead of a pathway to agreement.
Iran says it is enriching uranium only for civilian purposes.
Obama said international sanctions on Iran were having significant effects on the Iranian economy and would begin to "bite" harder in the summer.
"I have sent a message very directly to (Iran) publicly that they need to seize this opportunity of negotiations with the P5+1 to avert even worse consequences for Iran in the future," Obama said.
"I am determined not simply to contain (an) Iran that is in possession of a nuclear weapon, I am determined to prevent Iran from getting a nuclear weapon."
A Reuters/Ipsos poll Tuesday showed that 56 percent of Americans would support U.S. military action against Iran if there were evidence that Tehran is building nuclear weapons.
(Editing by Bill Trott)