SINGAPORE, SHANGHAI AND BEIRUT — Presenting a united front on Iran's nuclear energy program, President Obama and Russian counterpart Dmitry Medvedev warned Sunday that they were losing patience with Tehran and wouldn't wait much longer for it to accept a proposal to resolve the dispute.

After an hourlong meeting in Singapore on the sidelines of the Asia-Pacific Economic Cooperation summit, the two leaders expressed dissatisfaction with Iran's response to a proposal to ship its enriched uranium abroad to be refined further for use in an Iranian reactor to produce medical isotopes. In Geneva last month, Iran agreed to the deal in principle, but Western officials said Iranian leaders have since put up obstacles.

"Unfortunately, so far at least, Iran appears to have been unable to say yes to what everyone acknowledges is a creative and constructive approach," Obama said after meeting with Medvedev. Obama called the offer to Iran a fair one.

If Tehran refuses, Obama said, "the alternative would be an approach that would involve increasing pressure on Iran to meet its international obligations."

Medvedev, for his part, said, "We're still not satisfied with the pace of advancement of the process."

Obama moved on to China after the meeting in Singapore, and at a town hall-style meeting in Shanghai today he tried to reassure an audience of more than 400 students that Washington does not seek to continue the Cold War-era policy of containment. But he declared that the U.S. would speak out for human rights.

Students from eight Shanghai universities were present. They had to apply to attend and were interviewed and chosen by department heads.

Russian leaders other than Medvedev, notably Prime Minister Vladimir Putin, have been more averse to the prospect of increased economic sanctions. U.S. officials said Iran had till year-end to act.

The U.S. and Russia hope the threat of more sanctions will be enough to win agreement from Iran, and officials said the two leaders discussed a timetable for possible actions.

The U.S. and many other Western powers allege that Iran is seeking to develop nuclear weapons; the outsourcing proposal would diminish Iran's stockpile of uranium below the level needed to make a single nuclear bomb if it were further enriched. Iran insists its nuclear development program is for civilian energy purposes only.