"Because the international community has applied so many sanctions, because we have employed so many of the options that are available to us to persuade Iran to take a different course, the window for solving this issue diplomatically is shrinking," Obama said Wednesday in a White House Rose Garden news conference with British Prime Minister David Cameron.
"I hope that the Iranian regime understands that this is their best bet for resolving this in a way that allows Iran to rejoin the community of nations and to prosper and feel secure themselves," Obama said.
He warned Tehran, which made an offer last month to resume the talks, not "to delay, to stall, to do a lot of talking but not actually move the ball forward" -- a practice he said Iran had "a tendency" to do in previous negotiations.
Tehran had no immediate comment.
The Russian business newspaper Kommersant reported Wednesday U.S. Secretary of State Hillary Clinton sent a message to Iran through Russian Foreign Minister Sergei Lavrov that negotiations with the global powers dealing with Iran's disputed nuclear program were the country's "last chance."
The newspaper quoted an unnamed Russian diplomatic source as saying Israeli pressure was forcing Washington to support a military option this year to stop Tehran from developing a nuclear bomb.
"The invasion will happen before the year's end," Kommersant quoted the source as saying.
The U.S. State Department did not comment on the story Wednesday.
Clinton met with Lavrov at the United Nations in New York Monday.
Israeli Prime Minister Binyamin Netanyahu said last week after meeting with Obama in Washington Israel could attack Iranian nuclear facilities within months, but he added he hoped "there won't be a war."
U.S., Israeli and European officials, supported by U.N. weapons inspectors, maintain Iran plans to build nuclear weapons. Tehran insists its nuclear program is for peaceful civilian uses only.
Israel has repeatedly said it would not let Iran reach nuclear-weapons capacity and has declared it has an option to launch a military strike against Iran's nuclear facilities as a last resort.
Obama said Wednesday he has "sent a message very directly to [Iran] publicly that they need to seize this opportunity of negotiations with the P5 plus 1 to avert even worse consequences for Iran in the future."
The P5 plus 1 are the five permanent members of the U.N. Security Council -- the United States, Russia, China, France and Britain -- plus Germany.
Cameron said he agreed with Obama on pressuring Iran into "a peaceful outcome." But he said: "Nothing is off the table. That is essential for the safety of the region and the wider world."
An Iranian political and human-rights activist living in the United States told the Israeli newspaper Haaretz she saw a third option -- helping bring about a regime change in Iran by aiding the opposition.
"The regime can be brought down, and we have begged and begged and begged for some assistance for us to do it ourselves, and we sure can but seems like it's the idiocy of the people who cannot think outside the box," Banafsheh Zand-Bonazzi said, adding, "No one in Iran wants to get bombed.".
A Washington Post-ABC News poll indicated 64 percent of Americans say it's more important to see if economic sanctions work, even if it gives Iran more time to advance its nuclear program, while 26 percent support attacking Iran soon. Ten percent offered no opinion.
A separate national public opinion poll found 40 percent of Americans believe the United States will likely have a military conflict with Iran this year. The Poll Position survey found 39 percent said an armed conflict was unlikely and 21 percent offered no opinion.
The Washington Post-ABC News phone poll of 1,003 adults has a 4 percentage-point margin of error. The Poll Position phone survey of 1,152 registered voters has a 3-point margin of error.
A third poll, released Thursday, found 50 percent of Israelis believe Israel shouldn't attack Iranian nuclear reactors, even if diplomatic attempts fail. Forty-three percent support such a strike, the Knesset Channel poll found.
While 78 percent of the Israeli public believes a military strike would postpone Iran's nuclear-weapons acquisition, only 16 percent believe it will eliminate its nuclear capability altogether, the poll indicated.
A summary of the poll findings did not include the number of people surveyed or the margin of error. Previous Knesset Channel polls had margins of error of about 4.5 percentage points.